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
import sys
input=sys.stdin.readline
sys.setrecursionlimit(300000)
from collections import deque
def beauty(n,m,s,x,y):
    #print("hi")
    degree=[0 for i in range(n)]
    graph={i:[] for i in range(n)}
    for i in range(m):
      graph[x[i] - 1].append(y[i] - 1)
      degree[y[i] - 1] += 1
    q=deque()
    for i in range(n):
      if degree[i]==0:
        q.append(i)
    count=0
    ans=0
    # print(degree)
    dp=[[0 for i in range(26)] for i in range(n)]
 
    while count<n and q:
      x=q.popleft()
      count+=1
      # print(ord(s[x])-97)
      dp[x][ord(s[x])-97]+=1
      for i in graph[x]:
        for j in range(26):
          dp[i][j]=max(dp[i][j],dp[x][j])
        degree[i]-=1
        if degree[i]==0:
          q.append(i)
    # print(degree)
    if count!=n:
      return -1 
    else:
      ans=0
      for i in range(n):
        ans=max(ans,max(dp[i]))
      return ans
n,m= list(map(int,input().split()))
s=input()
x=[]
y=[]
for i in range(m):
    a,b=list(map(int,input().split()))
    x.append(a)
    y.append(b)
print(beauty(n,m,s,x,y))