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 dfs(node):
    vis[node] = True
    char = {chr(i):0 for i in range(97+26)}
    char[s[node-1]] += 1
    for child in adj[node]:
        if not vis[child]:
            c = dfs(child)
            for ele in c:
                char[ele] += c[ele]
    return char
def toposort(n,indg,adj):
    q = [];res =[]
    for i in range(1,n+1):
        if indg[i] == 0:
            q.append(i)
    while q:
        curr = q.pop(0)
        res.append(curr)
        for child in adj[curr]:
            indg[child] -= 1
            if indg[child] == 0:
                q.append(child)
    return len(res)
        
n,m = map(int,input().split())
s = input()
adj = {i:[] for i in range(1,n+1)}
indg = {node:0 for node in range(1,n+1)}
for edge in range(m):
    u,v = map(int,input().split())
    indg[v]+=1
    adj[u].append(v)
## checking for cycles as answer will become infinite
#print(adj)
if toposort(n,indg,adj) != n:
    print(-1)
else:
    vis = [False]*(n+1);ans = -1
    for node in range(1,n+1):
        if not vis[node]:
            c1 = dfs(node)
    ans = max(ans,max(c1.values()))
    print(ans)

