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 sys import stdin, stdout 

from collections import deque

n,m=[int(_) for _ in stdin.readline().split()]
s=input()
x,y=[],[]
for _ in range(m):
	a,b=map(int,stdin.readline().strip().split())
	x.append(a)
	y.append(b)

l=list(s)
D=[0 for _ in range(n)]
G={_:[] for _ in range(n)}
for _ in range(m):
	a,b=x[_],y[_]
	a-=1
	b-=1
	G[a].append(b)
	D[b]+=1
q=deque()
for _ in range(n):
	if D[_]==0:
		q.append(_)

c=0
ans=0
DP=[[0 for _ in range(26)] for _ in range(n)]
while c<n and q:
	x=q.popleft()
	c+=1
	DP[x][ord(l[x])-97]+=1
	for _ in G[x]:
		for __ in range(26):
			DP[_][__]=max(DP[_][__],DP[x][__])
		D[_]-=1
		if D[_]==0:
			q.append(_)
			
if c!=n:
	print(-1)
else:
	for _ in range(n):
		ans=max(ans,max(DP[_]))
	print(ans)