// problem D
#include <bits/stdc++.h>
using namespace std;

const int N = 3e5 + 1;

int n , m;

char arr[N];
bool vis[N];
bool loop[N];
bool inrecur[N];

vector < int > adj[N];

bool dfs(int u) {
	int to = (int) adj[u].size();
	for(int i = 0; i < to; ++i) {
		int nxt = adj[u][i];
		if(!vis[nxt]) {
			vis[nxt] = true;
			inrecur[nxt] = true;
			if(dfs(nxt)) {
				loop[u] = true;
				return true;
			}
		} else {
			if(inrecur[nxt]) {
				loop[u] = true;
				return true;
			}
		}
	}
	inrecur[u] = false;
	return false;
}
int dp[N][26];

void go(int u) {
	int to = (int) adj[u].size();
	if(!to) {
		dp[u][arr[u] - 'a'] = 1;
		return;
	}
	for(int i = 0; i < to; ++i) {
		go(adj[u][i]);
		for(int j = 0; j < 26; ++j) {
			dp[u][j] = max(dp[u][j] , (j == (arr[u] - 'a')) + dp[adj[u][i]][j]);
		}
	}
}

int solve() {
	int ans = 0;
	memset(dp , -1 , sizeof dp);
	for(int i = 1; i <= n; ++i) {
		go(i);
		for(int j = 0; j < 26; ++j) {
			ans = max(ans , dp[i][j]);
		}
	}
	return ans;
}

int main() {
	scanf("%d %d" , &n , &m);
	scanf("%s" , arr + 1);
	while(m--) {
		int u , v;
		scanf("%d %d" , &u , &v);
		adj[u].push_back(v);
	}
	bool has_loop = false;
	for(int i = 1; i <= n; ++i) {
		if(!vis[i]) {
			vis[i] = true;
			inrecur[i] = true;
			if(dfs(i)) {
				has_loop = true;
				break;
			}
		}
	}
	if(has_loop) {
		printf("-1\n");
		return 0;
	}
	printf("%d\n" , solve());
	return 0;
}