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 solve(nodes, vals, edges):
    
    #Only take the value of the max adjacent letter
    
    max_arr = {key: 0 for key in vals[0]}
    
    for idx, node in enumerate(nodes):
        vals[idx][node] += 1
        if idx and edges[idx] != -1:
            temp = {key: 0 for key in vals[idx].keys()}
            for edge in edges[idx]:
                for key in vals[edge]:
                    if vals[edge][key] > temp[key]:
                        temp[key] = vals[edge][key]
            vals[idx] = {key:vals[idx][key] + temp[key] for key in temp} 
            for key in vals[idx]:
                if max_arr[key] <= vals[idx][key]:
                    max_arr[key] = vals[idx][key]
        
    
    ret_val = max(max_arr.values())
    return ret_val

if __name__=="__main__":
    
    contents = []
    while True:
        line = input()
        if line:         
            contents.append(line)
        else:
            break

#     contents = ['6 6',
# 'xzyabc',
# '1 2',
# '3 1',
# '2 3',
# '5 4',
# '4 3',
# '6 4']
    
    vals = [{x : 0 for x in set(contents[1])} for node in contents[1]]
    
    
    edges = {i : -1 for i in range(len(contents[1]))}
    
    for e in contents[2:]:
        cur = list(map(lambda x: int(x)-1, e.split(' ')))
        
        if edges[cur[1]] == -1:
            edges[cur[1]] = []
        
        edges[cur[1]].append(cur[0])
        
    
    print(solve(contents[1], vals, edges))