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
def beauty(n,m,s,x,y):
    q=deque()
    res=0
    dg=[0 for i in range(n)]
    cnt=0

    graph=defaultdict(list)
    for i in range(m):
        l,k=x[i],y[i]
        l=l-1
        k=k-1
        graph[l].append(k)
        dg[k]=dg[k]+1
    mt=[[0 for i in range(26)] for i in range(n)]
    for i in range(n):
            if dg[i]==0:
                    q.append(i)
    while(cnt <n and q):
        t=q.popleft()
        cnt=cnt+1
        mt[t][ord(s[t])-97]+=1
        for i in graph[t]:
            for j in range(26):
                mt[i][j]=max(mt[i][j],mt[t][j])
            dg[i]-=1
            if dg[i]==0:
                q.append(i)
    if(cnt!=n):
        print(-1)
    else:
        for i in range(0,n):
            res=max(res,max(mt[i]))
        print(res)



n,m=map(int,input().split())
arr=list(input())
#xt=list(map(int,input().split()))
#yt=list(map(int,input().split()))
x,y=[],[]
for i in range(m):
    a,b=map(int,input().split())
    x.append(a)
    y.append(b)
beauty(n,m,arr,x,y)'''



from collections import defaultdict,deque
import sys
input=sys.stdin.readline

def beauty(n,m,l,x,y):
    deg=[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)
        deg[b]+=1
    q=deque()
    for i in range(n):
        if deg[i]==0:
            q.append(i)
    count=0
    ans=0
    dp=[[0 for i in range(26)] for i in range(n)]
    while count<n and q:
        und=q.popleft()
        count+=1
        dp[und][ord(l[und])-97]+=1
        for i in graph[und]:
            for j in range(26):
                dp[i][j]=max(dp[i][j],dp[und][j])
            deg[i]-=1
            if deg[i]==0:
                q.append(i)
    if count!=n:
        print(-1)
    else:
        ans=0
        for i in range(n):
            ans=max(ans,max(dp[i]))
        print(ans)

n,m=map(int,input().split())
l=list(input())

x=[]
y=[]
for i in range(m):
	a,b=map(int,input().split())
	x.append(a)
	y.append(b)

beauty(n,m,l,x,y)
