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
class Graph:
	def __init__(self,n):
		self.n = n + 1
		self.graph = {i:[] for i in range(n)}
		self.degree = [0 for i in range(n)]

	def add_edge(self,src,dest):
		self.graph[src].append(dest)
		self.degree[dest] += 1

	def compute(self,s,x,y):
		degree = self.degree
		graph = self.graph
		from collections import deque
		q = deque()
		for i in range(m):
			x[i] -= 1
			y[i] -= 1
			graph[x[i]].append(y[i])
			degree[y[i]] += 1

		for i in range(n):
			if degree[i] == 0:
				q.append(i)
		count = 0
		ans = 0

		dp = [[0 for i in range(26)] for i in range(n)]
		while count < n and q:
			a= q.popleft()
			count += 1
			dp[a][ord(s[a])-97] += 1
			for i in graph[a]:
				for j in range(26):
					dp[i][j] = max(dp[i][j],dp[a][j])
				degree[i] -= 1
				if degree[i] == 0:
					q.append(i)
		if count != n:
			return -1
		else:
			ans = 0
			for i in range(n):
				ans = max(ans,max(dp[i]))
			return ans
 
		







if __name__ == '__main__':
	n,m = int(input())
	s = input()
	x = []
	y = []
	for i in range(m):
		a,b = map(int,input().split())
		x.append(a)
		y.append(b)
	g = Graph(n)
	#print(g.graph,g.degree)
	print(g.compute(s,x,y))
