No to Palindromes!

给你一个长度为N的串，其中只包含前p个英文字母，保证输入的是一个子序列中没有长度为2或者是长度大于2的回文串。 让你找到一个比原字符串字典序大的第一个也满足:子序列中没有长度为2或者是长度大于2的回文串的解。如果不存在，输出NO。

Paul hates palindromes. He assumes that string $ s $ is tolerable if each its character is one of the first $ p $ letters of the English alphabet and $ s $ doesn't contain any palindrome contiguous substring of length 2 or more. Paul has found a tolerable string $ s $ of length $ n $ . Help him find the lexicographically next tolerable string of the same length or else state that such string does not exist.

The first line contains two space-separated integers: $ n $ and $ p $ ( $ 1<=n<=1000 $ ; $ 1<=p<=26 $ ). The second line contains string $ s $ , consisting of $ n $ small English letters. It is guaranteed that the string is tolerable (according to the above definition).

If the lexicographically next tolerable string of the same length exists, print it. Otherwise, print "NO" (without the quotes).

3 3
cba

NO

3 4
cba

cbd

4 4
abcd

abda

String $ s $ is lexicographically larger (or simply larger) than string $ t $ with the same length, if there is number $ i $ , such that $ s_{1}=t_{1} $ , ..., $ s_{i}=t_{i} $ , $ s_{i+1}&gt;t_{i+1} $ . The lexicographically next tolerable string is the lexicographically minimum tolerable string which is larger than the given one. A palindrome is a string that reads the same forward or reversed.