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给你一个有向图，每个点有一个小写字母权值 定义一条路径的权值是这条路径出现次数最多的字母的个数 求出这个图权值最大权值的路径的权值。 可能有自环。。 Translated by 远航之曲

You are given a graph with $ n $ nodes and $ m $ directed edges. One lowercase letter is assigned to each node. We define a path's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is $ 3 $ . Your task is find a path whose value is the largest.

The first line contains two positive integers $ n,m $ ( $ 1<=n,m<=300000 $ ), denoting that the graph has $ n $ nodes and $ m $ directed edges. The second line contains a string $ s $ with only lowercase English letters. The $ i $ -th character is the letter assigned to the $ i $ -th node. Then $ m $ lines follow. Each line contains two integers $ x,y $ ( $ 1<=x,y<=n $ ), describing a directed edge from $ x $ to $ y $ . Note that $ x $ can be equal to $ y $ and there can be multiple edges between $ x $ and $ y $ . Also the graph can be not connected.

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

5 4
abaca
1 2
1 3
3 4
4 5

3

6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4

-1

10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7

4

In the first sample, the path with largest value is $ 1→3→4→5 $ . The value is $ 3 $ because the letter 'a' appears $ 3 $ times.