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):
    adj = [[] for _ in range(n + 1)]
    vis = [0 for _ in range(n + 1)]
    dp = [[0 for j in range(n + 1)] for i in range(26)]
    ans = 0
    cycle = False
    a = s

    def dfs(node):
        nonlocal ans, cycle
        vis[node] = 1
        dp[ord(a[node - 1]) - ord('a')][node] += 1
        for child in adj[node]:
            if vis[child] == 1:
                cycle = True
            elif vis[child] == 0:
                dfs(child)
            for i in range(26):
                dp[i][node] = max(dp[i][node], (a[node - 1] == chr(i + ord('a'))) + dp[i][child])
        for i in range(26):
            ans = max(ans, dp[i][node])
        vis[node] = 2

    for i in range(m):
        adj[x[i]].append(y[i])

    for i in range(1, n + 1):
        if vis[i] == 0:
            dfs(i)

    if cycle:
        return -1
    else:
        return ans

n = int(input().strip())
m = int(input().strip())
s = input().strip()
x = [int(i) for i in input().strip().split()]
y = [int(i) for i in input().strip().split()]
print(beauty(n, m, s, x, y))
