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 isCyclicUtil(v, visited, recStack):
        visited[v] = True
        recStack[v] = True
        for neighbour in graph[v]:
            # print(neighbour, visited)         
            if visited[neighbour] == False:
                if isCyclicUtil(neighbour, visited, recStack) == True:
                    return True
            elif recStack[neighbour] == True:
                return True
        recStack[v] = False
        return False

def isCyclic():
    visited = [False] * (V + 1)
    recStack = [False] * (V + 1)
    for node in range(V+1):
        if visited[node] == False:
            if isCyclicUtil(node,visited,recStack) == True:
                return True
    return False

# def dfs(node):
#     vis[node]=True
#     dic[val[node]]+=1
#     for child in graph[node]:
#         dfs(child)

def dfs(node, D):
    global mx
    vis[node]=True
    if val[node] not in D:
        D[val[node]]=1
    else:
        D[val[node]]+=1
    check=False
    for child in graph[node]:
        check=True
        dfs(child, dict(D))
    if check==False:
        for i in D:
            mx=max(mx, D[i])

# for t in range(int(input())):
V, M=map(int,input().split())
from collections import defaultdict
graph=defaultdict(list)
S=input().strip()
val=dict()
for i in range(len(S)):
    val[i+1]=S[i]
#     A=list(map(int,input().split()))
#     B=list(map(int,input().split()))
for i in range(M):
    U,L=map(int,input().split())
    graph[U].append(L)
# print(graph)
if isCyclic():
    print(-1)
else:
    mx= -1<<64
    vis=[False for i in range(V+1)]
    for i in range(1, V+1):
        if vis[i]==False:
            dic=defaultdict(int)
            dfs(i, dict())
    print(mx)