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
from sys import stdin, stdout
from types import GeneratorType

def bootstrap(f, stack=[]):
    def wrappedfunc(*args, **kwargs):
        if stack:
            return f(*args, **kwargs)
        else:
            to = f(*args, **kwargs)
            while True:
                if type(to) is GeneratorType:
                    stack.append(to)
                    to = next(to)
                else:
                    stack.pop()
                    if not stack:
                        break
                    to = stack[-1].send(to)
            return to
    return wrappedfunc

n, m = [int(x) for x in stdin.readline().split()]
s = stdin.readline().strip()

self_loop = False
neighbours = {i:[] for i in range(n)}
edges = set()
roots = [True]*n

for i in range(m):
    v, w = [int(x)-1 for x in stdin.readline().split()]
    if v == w:
        self_loop = True
    if (v,w) not in edges:
        edges.add((v,w))
        neighbours[v].append(w)
        roots[w] = False

if self_loop:
    stdout.write('-1\n')

else:
    status = [[0]*27 for i in range(n)]
    for i in range(n):
        status[i][ord(s[i])-96] = 1

    has_loop = [False]
    answer = [1]

    @bootstrap
    def update(v):
        if status[v][0] > 0:
            yield
        else:
            status[v][0] = 1
            for w in neighbours[v]:
                if status[w][0] == 1:
                    has_loop[0] = True
                    yield
                else:
                    yield update(w)
                    for j in range(1,27):
                        status[v][j] = max(status[v][j], status[w][j] + (ord(s[v])-96 == j))
                        answer[0] = max(answer[0], status[v][j])
            status[v][0] = 2
            yield

    for i in range(n):
        if roots[i]:
            update(i)

    if has_loop[0]:
        stdout.write('-1\n')
    else:
        stdout.write(str(answer[0])+'\n')
