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
from types import GeneratorType
 
def bootstrap(f, stack=[]):
      def wrappedfunc(*args, **kwargs):
            to = f(*args, **kwargs)
            if stack:
                  return to
            else:
                  while True:
                        if type(to) is GeneratorType:
                              stack.append(to)
                              to = next(to)
                        else:
                              stack.pop()
                        if not stack:
                              return to
                        to = stack[-1].send(to)
            return wrappedfunc
 
import sys
input=sys.stdin.readline
from collections import defaultdict as dd

n,m=map(int,input().split())
s=input()
d=dd(list)
lol=0
for i in range(m):
      u,v=map(int,input().split())
      if(u==v):
            lol=1
            print(-1)
            exit()
      d[u].append(v)

vis=[0]*(n+1)
rvis=[0]*(n+1)
cou=[0]
dp=[dd(int) for i in range(n+1)]
mx=[0]
h=[0]
#print(d)
@bootstrap
def dfs(u):
      if(h[0]==1):
            yield
      vis[u]=1
      rvis[u]=1
      for i in d[u]:
            if(h[0]==1):
                  yield
            if(vis[i]==0):
                  yield dfs(i)
                  if(h[0]==1):
                        yield            
            elif(rvis[i]==True):
                  h[0]=1
                  yield
      rvis[u]=0
      if(h[0]==1):
            yield
      for i in d[u]:
            for j in dp[i]:
                  dp[u][j]=max(dp[u][j],dp[i][j])
                  mx[0]=max(mx[0],dp[u][j])
      dp[u][s[u-1]]+=1
    #print(u,dp[u])
      mx[0]=max(mx[0],dp[u][s[u-1]])
      yield
#print(dp)
for i in range(1,n+1):
      if(vis[i]==0):
            cou=[0]
            dfs(i)
if(h[0]):
      print(-1)
else:
      print(mx[0])
        
