Substring

Description:

Input:

The first line contains two positive integers $$$n, m$$$ ($$$1 \leq n, m \leq 300\,000$$$), 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 $$$x, y$$$ ($$$1 \leq x, y \leq n$$$), 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:

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

Sample Input:

5 4
abaca
1 2
1 3
3 4
4 5

Sample Output:

3

Sample Input:

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

Sample Output:

-1

Sample Input:

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

Sample Output:

4

Note:

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