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.
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.
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.
5 4
abaca
1 2
1 3
3 4
4 5
3
6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4
-1
10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7
4
1 1
f
1 1
-1
10 50
ebibwbjihv
1 10
1 2
5 4
1 8
9 7
5 6
1 8
8 7
2 6
5 4
1 9
3 2
8 3
5 6
5 9
2 4
2 7
3 9
1 2
1 7
1 10
3 7
1 8
3 10
8 6
1 7
10 6
1 6
5 8
1 5
2 10
3 9
5 8
8 3
3 7
5 2
1 10
1 4
5 3
3 2
1 2
5 8
10 4
2 10
8 2
1 9
1 8
1 2
3 4
1 8
2
13 37
dwpzcppjmhkmz
2 6
3 6
6 7
6 7
6 7
6 7
6 8
6 8
6 8
6 8
4 6
4 6
5 6
4 6
4 6
6 9
6 9
6 10
6 10
6 10
6 10
4 6
1 6
1 6
10 11
6 11
1 6
6 12
6 12
6 12
6 13
6 13
6 13
6 13
3 6
2 6
2 6
3
5 8
tetqw
2 1
4 4
5 5
5 2
4 5
1 5
1 5
1 1
-1
5 8
reeet
4 3
2 5
4 2
2 4
4 2
5 2
3 3
3 4
-1
#include<bits/stdc++.h>

typedef long long int ll;
typedef unsigned long long int ull;

#define dbg printf("in\n")
#define nl printf("\n");
#define N 300110
#define inf 100000000
#define pp pair<int,int>

#define sf(n) scanf("%d", &n)
#define sff(n,m) scanf("%d%d",&n,&m)
#define sfl(n) scanf("%I64d", &n)
#define sffl(n,m) scanf("%I64d%I64d",&n,&m)

#define pf(n) printf("%d\n",n)
#define pfl(n) printf("%I64d ",n)
#define pfs(s) printf("%s",s)

#define pb push_back
#define pp pair<int,int>

using namespace std;

int *color, dp[N][30];
bool vis[N];
string str;
vector<int> adj[N];

void dfs(int s)
{
    vis[s]=1;

    for(int e : adj[s])
    {
        if(!vis[e])
            dfs(e);

        for(int i=0;i<26;i++)
            dp[s][i]=max(dp[s][i],dp[e][i]);
    }

    dp[s][str[s-1]-'a']++;
}

void cycle(int s)
{
    color[s]=1;

    for(int e : adj[s])
    {
        if(!color[e])
            cycle(e);

        else if(color[e]==1)
        {
            pf(-1);
            exit(0);
        }

        else
            continue;
    }

    color[s]=2;
}

int main()
{
    freopen("in.txt", "r", stdin);

    int i,j,k;
    int n,m;
    int u,v;

    sff(n,m);
    cin>>str;

    for(i=0; i<m; i++)
    {
        sff(u,v);
        adj[u].pb(v);
    }

    color=new int[n+1];
    fill(color,color+n+1,0);

    for(i=1; i<=n; i++)
    {
        if(!vis[i])
            cycle(i);
    }

    delete color;

    memset(vis,0,sizeof(vis));

    int ans=0;
    for(i=1; i<=n; i++)
    {
        if(!vis[i])
        {
            dfs(i);
            for(j=0;j<26;j++)
                ans=max(ans,dp[i][j]);
        }
    }

    pf(ans);

    return 0;
}