Statement Update.

問題文が更新されました．
追加された文章は，「すぬけくんは，常に，どのランプが点灯しているかを知ることができます．」です．

The statement is updated.
The added sentence is "At every moment, Snuke knows which lamps are on."

What does Snuke knows?
if he knows which lamp is on at every moment, then he could figure out a p_i after he chooses it
if he doesn't know, then how can he choose a lamp "which is on"?

Snuke knows which lamps are on at every moment.
So he can figure out p_i after pressing Button i.

A Clarification.

S_i とは，文字列 S の i 文字目の文字が表す数です．

S_i means the number represented by the i-th character of the string S.

Statement Correction.

B 問題の問題文に，$N$ という変数が使われておりましたが，$|S|$ の間違いです．
問題文はすでに修正されています．

The problem statement of B has an undefined variable N, but it was |S|.
The statement is now fixed.