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
def beauty(n,m,s,x,y):
    edges={}
    for i in range(m):
        if edges.get(x[i]):
            edges[x[i]]+=[y[i]]
        else: edges[x[i]]=[y[i]]
    def maxi(lis):
        if lis==[]: return 0
        k={}
        ma=0
        for i in lis:
            kk=s[i-1]
            if k.get(kk) is None:
                k[kk]=1 
            else:
                k[kk]+=1 
            if k[kk]>ma:
                ma=k[kk]
        return ma 
    def path(node,done):
        if node in done:
            raise Exception
        if edges.get(node) is None:
            return[done+[node]]
        temp=[]
        for i in edges[node]:
            temp+=path(i,done+[node])
        return temp
    ma=0
    try:
        for i in range(1,n+1):
            for j in path(i,[]):
                mm=maxi(j)
                if mm>ma: ma=mm
        return ma
    except Exception:
        return -1

x = list(map(int, input().split()))
n=x[0]
m=x[1]
s=input().strip()
x = []
y=[]
for i in range(0,m):
    inp = list(map(int, input().split()))
    x.append(inp[0])
    y.append(inp[1])

print(beauty(n,m,s,x,y))