With 225 moves, this is a looong problem. Don't get tired!
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[19]KM[0.00]TM[]
PW[Weiß]PB[Schwarz]AW[qa][ac][cc][ec][fc][qc][dd][pd][qe][qf][cn][bo][do][go][io][jo][bp][pp][bq][nq][oq][qq][sq][cr][er][cs][ds]AB[gm][dn][en][hn][ho][cp][dp][cq][eq][ar][br][dr][bs]TR[cr][er][cs][ds]C[White to play and save the marked stones!]PL[W]
;W[dq]CR[dq]
;B[es]LB[aq:A]CR[es]C[Black has to prevent a white move at A]
;W[fs]CR[fs]
;B[dr]CR[dr]
;W[es]CR[es]
;B[fr]CR[fr]C[So black has no choice.....]
(;W[gs]CR[gs]
;B[gr]CR[gr]
;W[hs]CR[hs]
;B[hr]CR[hr]
;W[is]CR[is]
;B[ir]CR[ir]
;W[js]CR[js]
;B[jr]CR[jr]
;W[ks]CR[ks]
;B[kr]CR[kr]
;W[ls]CR[ls]
;B[lr]CR[lr]
;W[ms]CR[ms]
;B[mr]CR[mr]
;W[ns]CR[ns]
;B[nr]CR[nr]
;W[os]CR[os]
;B[or]CR[or]
;W[ps]CR[ps]
;B[pr]CR[pr]
;W[qs]CR[qs]
;B[qr]CR[qr]
;W[rs]CR[rs]
;B[rr]CR[rr]C[U]
;W[dq]CR[dq]
;B[ss]CR[ss]C[Finally, white got a chance to take the ko, but black gives one more atari...]
;W[sr]CR[sr]
;B[dr]CR[dr]C[.. and retakes the ko. ]
;W[ss]CR[ss]
;B[rq]CR[rq]
;W[sp]CR[sp]
;B[rp]CR[rp]
;W[so]CR[so]
;B[ro]CR[ro]
;W[sn]CR[sn]
;B[rn]CR[rn]
;W[sm]CR[sm]
;B[rm]CR[rm]
;W[sl]CR[sl]
;B[rl]CR[rl]
;W[sk]CR[sk]
;B[rk]CR[rk]
;W[sj]CR[sj]
;B[rj]CR[rj]
;W[si]CR[si]
;B[ri]CR[ri]
;W[sh]CR[sh]
;B[rh]CR[rh]
;W[sg]CR[sg]
;B[rg]CR[rg]
;W[sf]CR[sf]
;B[rf]CR[rf]
;W[se]CR[se]
;B[re]CR[re]
;W[sd]CR[sd]
;B[rd]CR[rd]
;W[sc]CR[sc]
;B[rc]CR[rc]
;W[sb]CR[sb]
;B[rb]CR[rb]
;W[dq]CR[dq]
;B[sa]CR[sa]C[You already know this :)]
;W[ra]CR[ra]
;B[dr]CR[dr]
;W[sa]CR[sa]
;B[qb]CR[qb]
;W[pa]CR[pa]
;B[pb]CR[pb]
;W[oa]CR[oa]
;B[ob]CR[ob]
;W[na]CR[na]
;B[nb]CR[nb]
;W[ma]CR[ma]
;B[mb]CR[mb]
;W[la]CR[la]
;B[lb]CR[lb]
;W[ka]CR[ka]
;B[kb]CR[kb]
;W[ja]CR[ja]
;B[jb]CR[jb]
;W[ia]CR[ia]
;B[ib]CR[ib]
;W[ha]CR[ha]
;B[hb]CR[hb]
;W[ga]CR[ga]
;B[gb]CR[gb]
;W[fa]CR[fa]
;B[fb]CR[fb]
;W[ea]CR[ea]
;B[eb]CR[eb]
;W[da]CR[da]
;B[db]CR[db]
;W[ca]CR[ca]
;B[cb]CR[cb]
;W[ba]CR[ba]
;B[bb]CR[bb]
;W[dq]CR[dq]
;B[aa]CR[aa]
;W[ab]CR[ab]
;B[dr]CR[dr]
;W[aa]CR[aa]
;B[bc]CR[bc]
;W[ad]CR[ad]
;B[bd]CR[bd]
;W[ae]CR[ae]
;B[be]CR[be]
;W[af]CR[af]
;B[bf]CR[bf]
;W[ag]CR[ag]
;B[bg]CR[bg]
;W[ah]CR[ah]
;B[bh]CR[bh]
;W[ai]CR[ai]
;B[bi]CR[bi]
;W[aj]CR[aj]
;B[bj]CR[bj]
;W[ak]CR[ak]
;B[bk]CR[bk]
;W[al]CR[al]
;B[bl]CR[bl]
;W[am]CR[am]
;B[bm]CR[bm]
;W[an]CR[an]
;B[bn]CR[bn]
;W[ao]CR[ao]
;B[aq]LB[ap:B][as:A]CR[aq]C[Black plays this way in order to prevent a second eye. Now, if black can play A, white will be dead even if he captures the five black stones. Therefore he must gain a liberty and then play B. ]
(;W[ep]CR[ep]
(;B[fp]CR[fp]
;W[eo]CR[eo]
(;B[dq]CR[dq]
;W[co]CR[co]
;B[fq]CR[fq]
;W[ap]CR[ap]
;B[fo]LB[as:A]CR[fo]C[White wants to capture at A, but before that he needs at least a ko threat or two liberties. Black prevents this as long as possible!]
;W[cm]CR[cm]
;B[dm]CR[dm]C[From now on it's plain sailing again.]
;W[cl]CR[cl]
;B[dl]CR[dl]
;W[ck]CR[ck]
;B[dk]CR[dk]
;W[cj]CR[cj]
;B[dj]CR[dj]
;W[ci]CR[ci]
;B[di]CR[di]
;W[ch]CR[ch]
;B[dh]CR[dh]
;W[cg]CR[cg]
;B[dg]CR[dg]
;W[cf]CR[cf]
;B[df]CR[df]
;W[ce]CR[ce]
;B[de]CR[de]
;W[cd]CR[cd]
;B[ed]CR[ed]
;W[dc]CR[dc]
;B[fd]CR[fd]
;W[gc]CR[gc]
;B[gd]CR[gd]
;W[hc]CR[hc]
;B[hd]CR[hd]
;W[ic]CR[ic]
;B[id]CR[id]
;W[jc]CR[jc]
;B[jd]CR[jd]
;W[kc]CR[kc]
;B[kd]CR[kd]
;W[lc]CR[lc]
;B[ld]CR[ld]
;W[mc]CR[mc]
;B[md]CR[md]
;W[nc]CR[nc]
;B[nd]CR[nd]
;W[oc]CR[oc]
;B[od]CR[od]
;W[pc]CR[pc]
;B[pe]CR[pe]
;W[qd]CR[qd]
;B[pf]CR[pf]
;W[qg]CR[qg]
;B[pg]CR[pg]
;W[qh]CR[qh]
;B[ph]CR[ph]
;W[qi]CR[qi]
;B[pi]CR[pi]
;W[qj]CR[qj]
;B[pj]CR[pj]
;W[qk]CR[qk]
;B[pk]CR[pk]
;W[ql]CR[ql]
;B[pl]CR[pl]
;W[qm]CR[qm]
;B[pm]CR[pm]
;W[qn]CR[qn]
;B[pn]CR[pn]
;W[qo]CR[qo]
;B[po]CR[po]
;W[qp]CR[qp]
;B[op]CR[op]
;W[pq]CR[pq]
;B[np]CR[np]
;W[mq]CR[mq]
;B[mp]CR[mp]
;W[lq]CR[lq]
;B[lp]CR[lp]
;W[kq]CR[kq]
;B[kp]CR[kp]
;W[jq]CR[jq]
;B[jp]CR[jp]
;W[iq]CR[iq]
;B[ip]CR[ip]
;W[hq]CR[hq]
;B[hp]CR[hp]
;W[as]CR[as]
;B[ar]CR[ar]
;W[br]CR[br]
;B[bs]LB[aq:A]CR[bs]C[Black successfully prevented white from getting two liberties (in which case white would be able to atari at A), but in the process white has gained a ko threat!]
(;W[fn]CR[fn]
;B[gp]CR[gp]
;W[as]LB[aq:A]CR[as]C[Eventually, white will capture at A and live. RIGHT CHOICE])
(;W[gp]CR[gp]
;B[fn]CR[fn]
;W[as]LB[aq:A]CR[as]C[Eventually, white will capture at A and live. RIGHT]))
(;B[fq]LB[dq:A]CR[fq]C[This is a mistake by black, A is the only move!]
;W[dq]CR[dq]C[White lives. RIGHT CHOICE]))
(;B[eo]CR[eo]
;W[fp]CR[fp]
;B[co]CR[co]
;W[dq]CR[dq]
;B[do]CR[do]
;W[fq]TR[eo]CR[fq]C[ White is alive, but only because black made a mistake: he can't atari with the marked stone! RIGHT CHOICE]))
(;W[dq]CR[dq]
;B[ep]CR[ep]
(;W[ap]CR[ap]
;B[co]CR[co]
;W[as]CR[as]
;B[ar]CR[ar]
;W[br]CR[br]
;B[bs]LB[as:A]TR[dq]CR[bs]C[Since there is no ko threat, white is dead - black will connect at A with his next move. Taking the ko (the marked stone) was a mistake! ])
(;W[co]CR[co]
;B[as]CR[as]C[There is only one eye in the corner, and it's impossible to gain another in the center - white is dead. Taking the ko (the marked stone) was a mistake! ])))
(;W[dq]CR[dq]
;B[gs]CR[gs]C[Obviously, white can't take the ko. That is, for the moment!]))