String Power

Chef has been recently studying strings and probabilities and he came up with a problem for you. Consider an alphabet with $N$ characters (denoted by $c_1, c_2, \ldots, c_N$). For each valid $i$, the character $c_i$ has a *power* $a_i$; the power of a string that contains only characters from this alphabet is the sum of their powers (if a character occurs multiple times, its power also occurs i