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):

    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(s[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):
    g,h=map(int,input().split())
    x.append(g)
    y.append(h)
print(beauty(n,m,s,x,y))