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
import queue

def topsort():
    q = queue.Queue()
    for i in range(1, n + 1):
        if id[i] == 0:
            q.put(i)
    tot = 0
    while not q.empty():
        x = q.get()
        tot += 1
        for i in hd[x]:
            if id[to[i]] == 0:
                q.put(to[i])
            id[to[i]] -= 1
    return tot == n

def dp(x):
    if vis[x]:
        return g[x]
    vis[x] = True
    g[x] = 0
    for i in hd[x]:
        g[x] = max(g[x], dp(to[i]))
    g[x] += f[x]
    return g[x]

n, m = map(int, input().split())
s = input()
to = [0] * (m + 1)
ne = [0] * (m + 1)
hd = [[] for _ in range(n + 1)]
id = [0] * (n + 1)
for i in range(1, m + 1):
    u, v = map(int, input().split())
    to[i] = v
    hd[u].append(i)
    id[v] += 1

ans = 0
if not topsort():
    print(-1)
else:
    for i in range(26):
        vis = [False] * (n + 1)
        for j in range(1, n + 1):
            f[j] = (ord(s[j - 1]) - ord('a') == i)
        for j in range(1, n + 1):
            ans = max(ans, dp(j))
    print(ans)
