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.

from collections import defaultdict,deque
import sys
input = sys.stdin.readline
n,m = map(int,input().split())
s = input()
d=defaultdict(list)
degree = [0 for i in range(n)]
for i in range(m):
    a,b = map(int,input().split())
    d[a-1].append(b-1)

    degree[b-1]+=1

q = deque()

for i in range(n):
    if degree[i]==0:
        q.append(i)
dp = [[0 for i in range(26)]for j in range(n)]
c=0
while q:
    p = q.popleft()
    dp[p][ord(s[p])-ord("a")]+=1
    c+=1
    for i in (d[p]):
        for j in range(26):
            dp[i][j]=max(dp[i][j],dp[p][j])
        degree[i]-=1
        if degree[i]==0:
            q.append(i)
if c!=n:
    print(-1)
else:
    ans = 0
    for i in range(n):
        ans = max(ans,max(dp[i]))
    print(ans)
