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.

Input

The first line contains two positive integers n, m (1 ≤ n, m ≤ 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 ≤ x, y ≤ 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.

Output

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

Examples

Input

5 4
abaca
1 2
1 3
3 4
4 5


Output

3


Input

6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4


Output

-1


Input

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


Output

4

Note

In the first sample, the path with largest value is 1 → 3 → 4 → 5. The value is 3 because the letter 'a' appears 3 times.
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()
