Mutation Detection in Protein Sequences

In this problem, you are given two strings representing protein sequences. The first string is the original protein sequence and the second is a mutated version where a single amino acid (represented by one character) may differ from the original. Your task is to identify the mutated amino acid. If no mutation is present, output None.

Formally, let the original sequence be (S_1) and the mutated sequence be (S_2), where (S_1, S_2 \in \Sigma^n) and (\exists!; i ; (1 \leq i \leq n) ) such that (S_1[i] \neq S_2[i]). If no such (i) exists, print None.

inputFormat

The input consists of two lines read from standard input (stdin):
1. The first line contains the original protein sequence.
2. The second line contains the mutated protein sequence.
Both sequences have the same length and consist of uppercase letters.

outputFormat

Output a single line to standard output (stdout) containing the mutated amino acid character. If there is no mutation, output None.## sample

MAYKD
MAXKD

X