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,Counter
import sys
import bisect
import math
mod=1000000007
input=sys.stdin.readline
def beauty(n,m,z,x,y):
	l=z
	degree=[0 for i in range(n)]
	graph=defaultdict(list)
	for i in range(m):
		a,b=x[i],y[i]
		a-=1
		b-=1
		graph[a].append(b)
		degree[b]+=1
	q=deque()
	for i in range(n):
		if degree[i]==0:
			q.append(i)
	count=0
	ans=0
	# print(degree)
	dp=[[0 for i in range(26)] for i in range(n)]
	while count<n and q:
		x=q.popleft()
		count+=1
		# print(ord(l[x])-97)
		dp[x][ord(l[x])-97]+=1
		for i in graph[x]:
			for j in range(26):
				dp[i][j]=max(dp[i][j],dp[x][j])
			degree[i]-=1
			if degree[i]==0:
				q.append(i)
	# print(degree)
	if count!=n:
		return -1
	else:
		ans=0
		for i in range(n):
			ans=max(ans,max(dp[i]))
		return ans
		
n,m = map(int,input().split())
s=input()
x=[]
y=[]
for i in range(m):
	u,v=map(int,input().split())
	x.append(u)
	y.append(v)
print(beauty(n,m,s,x,y))