Balance the Game Board

A token sits at one of the positions 0..N, and the letters lie between adjacent positions. Each step flips the a/b letter the token crosses. Find the minimum number of steps needed to make the counts of the two letters equal, or −1 if it is impossible.

输入格式

The first line contains the letter string L (1..100, only a/b), and the second line contains start (0..N).

输出格式

Output a single integer, the answer.

样例 1

输入：

aaabab
0

输出：

1

样例 2

输入：

aaabab
6

输出：

5

样例 3

输入：

ababa
1

输出：

-1

样例说明

Example 1: moving right across the first a turns the string into baabab, with 3 of each letter; the answer is 1.

Example 2: moving left 5 steps flips indices 1..5, giving abbaba with 3 of each letter; no shorter move balances the letters, so the answer is 5.

Example 3: the total number of letters, 5, is odd, so they cannot be split into equal counts; output −1.