write a go solution for Description:
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.

Input Format:
The first line contains two positive integers n,m (1<=n,m<=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<=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 Format:
Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

Note:
In the first sample, the path with largest value is 1to3to4to5. The value is 3 because the letter 'a' appears 3 times.. Output only the code with no comments, explanation, or additional text.