#include<cstdio>

#include<algorithm>

#include<cstring>

#include<iostream>

#include<cmath>

#include<cctype>

#include<set>

#include<queue>

#include<vector>

#include<map>

using namespace std;

typedef long long LL;

inline int read() {

    int x=,f=;char ch=getchar();for(;!isdigit(ch);ch=getchar())if(ch=='-')f=-;

    for(;isdigit(ch);ch=getchar())x=x*+ch-'';return x*f;

}

const int N = ;

struct Edge { int to, nxt; } e[N];

int f[N], head[N], deg[N], q[N], d[N], En;

char s[N];

bool vis[N];

void add_edge(int u,int v) {

    ++En; e[En].to = v, e[En].nxt = head[u]; head[u] = En;

    deg[v] ++;

}

int main() {

    int n = read(), m = read();

    scanf("%s", s + );

    for (int i = ; i <= m; ++i) {

        int u = read(), v = read();

        add_edge(u, v);

    }

    int L = , R = ;

    for (int i = ; i <= n; ++i) {

        d[i] = deg[i];

        if (!deg[i]) q[++R] = i;

    }

    while (L <= R) {

        int u = q[L ++];

        vis[u] = ;

        for (int i = head[u]; i; i = e[i].nxt) {

            int v = e[i].to;

            if (!(--d[v])) q[++R] = v;

        }

    }

    for (int i = ; i <= n; ++i)

        if (!vis[i]) { puts("-1"); return ; }

    int ans = ;

    for (int c = 'a'; c <= 'z'; ++c) {

        L = , R = ;

        for (int i = ; i <= n; ++i) {

            f[i] = ;

            d[i] = deg[i];

            if (!deg[i]) {

                q[++R] = i;

                if (s[i] == c) f[i] = ;

            }

        }

        while (L <= R) {

            int u = q[L ++];

            ans = max(ans, f[u]);

            for (int i = head[u]; i; i = e[i].nxt) {

                int v = e[i].to;

                f[v] = max(f[v], f[u] + (s[v] == c)); // !!!

                if (!(--d[v])) q[++R] = v;

            }

        }

    }

    cout << ans;

    return ;

}