#include<bits/stdc++.h>
#define rep(i,j,k) for(register int i=j;i<=k;i++)
using namespace std;
template<typename T> void chkmax(T &x,T y){x=max(x,y);}
template<typename T> void read(T &num){
	char c=getchar();num=0;int f=1;
	while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}
	while(c>='0'&&c<='9'){num=(num<<3)+(num<<1)+(c^48);c=getchar();}
	num*=f;
}
template<typename T> void qwq(T x){
	if(x>9)qwq(x/10);
	putchar(x%10+'0');
}
template<typename T> void write(T x){
	if(x<0){x=-x;putchar('-');}
	qwq(x);putchar('\n');
}
char st[300010];

struct wzy{
	int next,vertice;
}edge[300010];
int head[300010];int len=0;
inline void add_edge(int x,int y){
	edge[++len].next=head[x];edge[len].vertice=y;head[x]=len;
	return;
}

bool flag=false;bool vis[300010];bool vis2[300010];
inline void dfs(int x){
	if(vis[x])return;
	if(vis2[x]){flag=true;return;}
	vis2[x]=true;
	for(int i=head[x];i;i=edge[i].next){
		dfs(edge[i].vertice);
	}
	vis2[x]=false;
	vis[x]=true;
	return;
}

int f[300010];
inline int dp(int x,char tmp){
	if(f[x]!=-1)return f[x];
	int re_value=0;
	for(int i=head[x];i;i=edge[i].next){
		chkmax(re_value,dp(edge[i].vertice,tmp));
	}
	f[x]=re_value+(tmp==st[x-1]);
	return f[x];
}

int main(){
	int n,m;read(n);read(m);
	scanf("%s",st);
	rep(i,1,m){
		int x,y;read(x);read(y);
		add_edge(x,y);
	}
	
	rep(i,1,n){
		dfs(i);
		if(flag==true)break;
	}
	if(flag==true){
		write(-1);return 0;
	}
	
	int ans=0;
	for(char i='a';i<='z';i++){
		memset(f,-1,sizeof(f));
		rep(j,1,n){
			chkmax(ans,dp(j,i));
		}
	}
	write(ans);
	return 0;
}