CF919D.Substring

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.

给你一个包含 $n$ 个节点和 $m$ 条有向边的图。每个节点被分配一个小写字母。我们定义一条路径的值为该路径上出现频率最高的字母的出现次数。例如，若某条路径上的字母序列为 "abaca"，则该路径的值为 $3$。你的任务是找出值最大的路径。

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.

第一行包含两个正整数 $n, m$（$1 \leq n, m \leq 300\,000$），表示该图有 $n$ 个节点和 $m$ 条有向边。

第二行包含一个仅由小写英文字母组成的字符串 $s$。其中第 $i$ 个字符表示分配给第 $i$ 个节点的字母。

接下来是 $m$ 行，每行包含两个整数 $x, y$（$1 \leq x, y \leq n$），描述一条从节点 $x$ 指向节点 $y$ 的有向边。注意：$x$ 可以等于 $y$，且 $x$ 与 $y$ 之间可能存在多条边。此外，该图可能不连通。

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

输出一行，包含一个整数，表示最大值。如果该值可以任意大，则输出 -1。

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.

在第一个样例中，价值最大的路径是 $1 \to 3 \to 4 \to 5$。其价值为 $3$，因为字母 'a' 出现了 $3$ 次。