SRM 597 Div2 500

Little Elephant from the Zoo of Lviv likes strings. You are given a string A and a string B of the same length. In one turn Little Elephant can choose any character of A and move it to the beginning of the string (i.e., before the first ch

Little Elephant from the Zoo of Lviv likes strings.

You are given a string A and a string B of the same length. In one turn Little Elephant can choose any character of A and move it to the beginning of the string (i.e., before the first character of A). Return the minimal number of turns needed to transform A into B. If it's impossible, return -1 instead.