#include <bits/stdc++.h>
using namespace std;
const int maxn = 3e5 + 5;
vector<int> g[maxn];
int n, m, f[maxn][26], in[maxn], ans;
char s[maxn];
bool vis[maxn];

bool tp_sort() { // 拓扑排序 没环返回true，有环返回false 
	queue<int> que;
	int cnt = 0;
	for (int i = 1; i <= n; i++)
		if (!in[i])
			que.push(i), cnt++;
	while (!que.empty()) {
		int u = que.front();
		que.pop();
		for (auto v : g[u])
			if (--in[v] == 0)
				que.push(v), cnt++;
	}
	return cnt == n;
}

void dfs(int u) {
	if (vis[u]) // 记忆化 
		return;
	vis[u] = true;
	f[u][s[u] - 'a'] = 1;
	for (auto v : g[u]) {
		dfs(v);
		for (int i = 0; i < 26; i++)
			f[u][i] = max(f[u][i], f[v][i] + (s[u] == 'a' + i));
	}
}

int main() {
	scanf("%d%d%s", &n, &m, s+1);
	for (int i = 0; i < m; i++) {
		int u, v;
		scanf("%d%d", &u, &v);
		g[v].push_back(u);
		in[u]++; // u的入度 
	}
	if (!tp_sort()) {
		puts("-1");
		return 0;
	}
	for (int i = 1; i <= n; i++) {
		dfs(i);
		for (int j = 0; j < 26; j++)
			ans = max(ans, f[i][j]);
	}
	printf("%d\n", ans);
	return 0;
}