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
input = stdin.readline
#// - remember to add .strip() when input is a string

seen = {}
def DFS(cur, path_letters, adj_lis):
  
  if len(adj_lis[cur]) == 0:

    all_letters.append(path_letters.copy())
    return 1

  next = adj_lis[cur]
  flag = False
  for i in next:
    seen[str(i)] = None
    if not str(i) in visited:

      path_letters.append(letters[i-1])
      visited[str(i)] = None
      DFS(i, path_letters, adj_lis)
      visited.pop(str(i))
      path_letters.pop()
      
    else:
  
      flag = True

  if flag: 
    all_letters.append(-1)
    return 1
  else:
    all_letters.append(path_letters.copy())

  return 1
  

  

  
    



n, m = map(int,input().split())

letters = input().strip()

adj_lis = []
for i in range(n+1):
  adj_lis.append([])


for i in range(m):

  node1, node2 = map(int,input().split())

  ar = adj_lis[node1]
  ar.append(node2)
  adj_lis[node1] = ar




all_letters = []
for i in range(1, n+1):

  if not (str(i) in seen or len(adj_lis[i]) == 0):

    #// start dfs

    
    path_letters = [letters[i-1]]

    
    visited = {}

    seen[str(i)] = None
    
    cur = i
    
    DFS(cur, path_letters, adj_lis)


#print(all_letters)

if -1 in all_letters:
  print(-1)
else:
  maxi = 0
  for i in all_letters:

    freq = {}
    for j in i:
      if j in freq:
        freq[j] += 1
      else:
        freq[j] = 1

    
    for i in freq:
      maxi = max(maxi,freq[i])
      
  print(maxi)