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.
# NOT MY CODE
# https://codeforces.com/contest/919/submission/80857731
 
from types import GeneratorType
 
def bootstrap(f, stack=[]):
    def wrappedfunc(*args, **kwargs):
        to = f(*args, **kwargs)
        if stack:
            return to
        else:
            while True:
                if type(to) is GeneratorType:
                    stack.append(to)
                    to = next(to)
                else:
                    stack.pop()
                    if not stack:
                        return to
                    to = stack[-1].send(to)
    return wrappedfunc
 
import sys
input=sys.stdin.readline
from collections import defaultdict as dd
'''
def iscyclic(g):
    for i in range(1,n+1):
        if(
'''
n,m=map(int,input().split())
s=input()
d=dd(list)
lol=0
for i in range(m):
    u,v=map(int,input().split())
    if(u==v):
        lol=1
        print(-1)
        exit()
    d[u].append(v)
'''
if(iscyclic(d)):
    lol=1
    print(-1)
    exit()
'''
vis=[0]*(n+1)
rvis=[0]*(n+1)
cou=[0]
dp=[dd(int) for i in range(n+1)]
mx=[0]
h=[0]
#print(d)
@bootstrap
def dfs(u):
    if(h[0]==1):
        yield
    vis[u]=1
    rvis[u]=1
    for i in d[u]:
        if(h[0]==1):
            yield
        if(vis[i]==0):
            yield dfs(i)
            if(h[0]==1):
                yield            
        elif(rvis[i]==True):
            h[0]=1
            yield
    rvis[u]=0
    if(h[0]==1):
        yield
    for i in d[u]:
        for j in dp[i]:
            dp[u][j]=max(dp[u][j],dp[i][j])
            mx[0]=max(mx[0],dp[u][j])
    dp[u][s[u-1]]+=1
    #print(u,dp[u])
    mx[0]=max(mx[0],dp[u][s[u-1]])
    yield
#print(dp)
for i in range(1,n+1):
    if(vis[i]==0):
        cou=[0]
        dfs(i)
if(h[0]):
    print(-1)
else:
    print(mx[0])
        