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 collections import defaultdict,deque
n,m = [int(x) for x in input().split()]

graph = defaultdict(list)
inDegree=defaultdict(int)
back = defaultdict(list)
dist=[[0 for _ in range(26)] for _ in range(n)]

string=input()

for _ in range(m):
    x,y=[int(x) for x in input().split()]
    graph[x].append(y)
    back[y].append(x)
    inDegree[y]+=1

level=deque([])

for i in range(1,n+1):
    if not inDegree[i]:
        level.append(i)

maxi=float("-inf")
ans=0
while level:
    node=level.popleft()
    for val in graph[node]:
        inDegree[val]-=1
        
        if not inDegree[val]:
            level.append(val)
    
    for val in back[node]:
        for char in range(26):
            if ord(string[node-1])-ord("a")==char:
                
                dist[node-1][char]=max(dist[node-1][char],(dist[val-1][char]+1))
            
            else:
                
                dist[node-1][char]=max(dist[node-1][char],(dist[val-1][char]))
            maxi=max(dist[node-1][char],maxi)
    if len(back[node])==0:
    
        char= ord(string[node-1])-ord("a")
        dist[node-1][char]+=1
    # print(dist[node-1],node,string[node-1])
    ans+=1
    
if ans==n:
    print(maxi)
else:
    print(-1)