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.
The first line contains two positive integers $n, m$ ($1 \leq n, m \leq 300\,000$), 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 $x, y$ ($1 \leq x, y \leq n$), 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.
In the first sample, the path with largest value is $1 \to 3 \to 4 \to 5$. The value is $3$ because the letter 'a' appears $3$ times.
5 4
abaca
1 2
1 3
3 4
4 5
3
6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4
-1
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
4
1 1
f
1 1
-1
10 50
ebibwbjihv
1 10
1 2
5 4
1 8
9 7
5 6
1 8
8 7
2 6
5 4
1 9
3 2
8 3
5 6
5 9
2 4
2 7
3 9
1 2
1 7
1 10
3 7
1 8
3 10
8 6
1 7
10 6
1 6
5 8
1 5
2 10
3 9
5 8
8 3
3 7
5 2
1 10
1 4
5 3
3 2
1 2
5 8
10 4
2 10
8 2
1 9
1 8
1 2
3 4
1 8
2
13 37
dwpzcppjmhkmz
2 6
3 6
6 7
6 7
6 7
6 7
6 8
6 8
6 8
6 8
4 6
4 6
5 6
4 6
4 6
6 9
6 9
6 10
6 10
6 10
6 10
4 6
1 6
1 6
10 11
6 11
1 6
6 12
6 12
6 12
6 13
6 13
6 13
6 13
3 6
2 6
2 6
3
5 8
tetqw
2 1
4 4
5 5
5 2
4 5
1 5
1 5
1 1
-1
5 8
reeet
4 3
2 5
4 2
2 4
4 2
5 2
3 3
3 4
-1
from collections import deque

def zero_func(nums):
    l=[]
    for i in range(nums):
        k=[]
        for j in range(0,26):
            k.append(0)
        l.append(k)
    return l
def degue_func(nums1,check):
    q = deque()
    for i in range(nums1):
        if check[i] == 0:
            q.append(i)
    return q


def beauty(n, m, s, x, y):
    Degree=[0]*n
    graph={i:[] for i in range(n)}
    c=0
    for i in range(m):
        x[i]-=1
        y[i]-=1
        graph[x[i]].append(y[i])
        Degree[y[i]]+=1
    check=Degree
    
    check_ans=degue_func(n,check)
    ans=zero_func(n)

    while c<n and check_ans:
        a=check_ans.popleft()
        c+=1
        ans[a][ord(s[a])-97]+=1
        for i in graph[a]:
            for j in range(0,26):
                ans[i][j]=max(ans[i][j],ans[a][j])
            Degree[i]-=1
            if Degree[i]==0:
                check_ans.append(i)
    if c!=n:
        return -1
    else:
        result=0
        for i in range(n):
            result=max(result,max(ans[i]))
        return result

n1,m1=map(int,input().split())
s1=input()
listX=[]
listY=[]
for t in range(m1):
    x1,y1=map(int,input().split())
    listX.append(x1)
    listY.append(y1)
x=beauty(n1,m1,s1,listX,listY)
print(x)