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 deque
n,m=map(int,input().split())
s=input()
graph=[ [] for i in range(n)]
freq=[[0]*26 for i in range(n)]
degree=[0]*n
degree[0]=0
for i in range(m):
    x,y=map(int,input().split())
    graph[x-1].append(y-1)
    degree[y-1]+=1
d=deque()
for i in range(n):
        if degree[i]==0:
            d.append(i)
            freq[i][ord(s[i])-97]+=1
c=0
m=1
while(d):
    c+=1
    x=d.popleft()
    for i in graph[x]:
        for j in range(26):
            freq[i][j]=max(freq[i][j],freq[x][j])
        degree[i]-=1
        if degree[i]==0:
            freq[i][ord(s[i])-97]+=1
            for j in range(26):
                m=max(m,freq[i][j])
            d.append(i)
if c!=n:
    print(-1)
else:
    print(m)
