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
def dfs(dp,node,edges,s,visited):
    stack = [node]
    while stack:
        node = stack[-1]
        if node not in visited:
            count = 0
            for kid in edges[node]:
                if kid in visited:
                    count += 1
                stack.append(kid)
 
            if count == len(edges[node]):
                for kid in edges[node]:
                    for i in xrange(26):
                        dp[node][i] = max(dp[node][i],dp[kid][i])
 
                dp[node][ord(s[node-1])-ord('a')] += 1
                visited.add(node)
                stack.pop()
        else:
            stack.pop()
                    
def dfs2(edges,node,visited,recursion_stack):
    stack = [node]
    while stack:
        node = stack[-1]
        visited.add(node)
        recursion_stack.add(node)
        count = 0
        for kid in edges[node]:
            if kid not in visited:
                stack.append(kid)
            elif kid in recursion_stack:
                return True
            else:
                count += 1
 
        if count == len(edges[node]):
            stack.pop()
            recursion_stack.remove(node)
 
    return False
            
 
def cycle(edges,n):
    visited = set()
    recursion_stack = set()
    for i in xrange(1,n+1):
        if i not in visited:
            if dfs2(edges,i,visited,recursion_stack):
                return True
 
    return False
 
def main():
    n,m = map(int,raw_input().split())
    edges = {}
    for i in xrange(1,n+1):
        edges[i] = []
 
    incoming = set()
    for i in xrange(1,n+1):
        incoming.add(i)
        
    s = raw_input()
    for i in xrange(m):
        a,b = map(int,raw_input().split())
        edges[a].append(b)
        if b in incoming:
            incoming.remove(b)
 
    if cycle(edges,n):
        print -1
        return
 
    dp = [[0 for i in xrange(26)] for j in xrange(n+1)]
    visited = set()
    for i in incoming:
        if i not in visited:
            dfs(dp,i,edges,s,visited)
 
    ans = 0
    for i in incoming:
        ans = max(ans,max(dp[i]))
 
    print ans
    
 
main()