Tsumego #070

(;GM[1]FF[4]AP[Drago:4.21]SZ[19]CA[UTF-8]AB[bq][bp][cp][dp][eq][ep][er]AW[cq][dq][ap][bo][bn][co][do][eo][fo][fp][fq][fr][fs])

★★
White to kill.

Quite a famous problem.