CF1272C Yet Another Broken Keyboard

Recently, Norge found a string $ s = s_1 s_2 \ldots s_n $ consisting of $ n $ lowercase Latin letters. As an exercise to improve his typing speed, he decided to type all substrings of the string $ s $ . Yes, all $ \frac{n (n + 1)}{2} $ of them! A substring of $ s $ is a non-empty string $ x = s[a \ldots b] = s_{a} s_{a + 1} \ldots s_{b} $ ( $ 1 \leq a \leq b \leq n $ ). For example, "auto" and "ton" are substrings of "automaton". Shortly after the start of the exercise, Norge realized that his keyboard was broken, namely, he could use only $ k $ Latin letters $ c_1, c_2, \ldots, c_k $ out of $ 26 $ . After that, Norge became interested in how many substrings of the string $ s $ he could still type using his broken keyboard. Help him to find this number.

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In the first example Norge can print substrings $ s[1\ldots2] $ , $ s[2\ldots3] $ , $ s[1\ldots3] $ , $ s[1\ldots1] $ , $ s[2\ldots2] $ , $ s[3\ldots3] $ , $ s[5\ldots6] $ , $ s[6\ldots7] $ , $ s[5\ldots7] $ , $ s[5\ldots5] $ , $ s[6\ldots6] $ , $ s[7\ldots7] $ .