```go
package main

import (
	"fmt"
	"io"
	"os"
)

func main() {
	data, err := io.ReadAll(os.Stdin)
	if err != nil || len(data) == 0 {
		return
	}
	ptr := 0

	readInt := func() int {
		for ptr < len(data) && (data[ptr] < '0' || data[ptr] > '9') {
			ptr++
		}
		if ptr >= len(data) {
			return 0
		}
		res := 0
		for ptr < len(data) && data[ptr] >= '0' && data[ptr] <= '9' {
			res = res*10 + int(data[ptr]-'0')
			ptr++
		}
		return res
	}

	readWord := func() string {
		for ptr < len(data) && data[ptr] <= ' ' {
			ptr++
		}
		if ptr >= len(data) {
			return ""
		}
		start := ptr
		for ptr < len(data) && data[ptr] > ' ' {
			ptr++
		}
		return string(data[start:ptr])
	}

	n := readInt()
	m := readInt()
	if n == 0 {
		return
	}

	s := readWord()

	adj := make([][]int, n)
	inDegree := make([]int, n)

	for i := 0; i < m; i++ {
		u := readInt() - 1
		v := readInt() - 1
		adj[u] = append(adj[u], v)
		inDegree[v]++
	}

	dp := make([][26]int32, n)
	queue := make([]int, 0, n)

	for i := 0; i < n; i++ {
		if inDegree[i] == 0 {
			queue = append(queue, i)
		}
		dp[i][s[i]-'a'] = 1
	}

	processed := 0
	var ans int32 = 0

	for len(queue) > 0 {
		u := queue[0]
		queue = queue[1:]
		processed++

		for c := 0; c < 26; c++ {
			if dp[u][c] > ans {
				ans = dp[u][c]
			}
		}

		for _, v := range adj[u] {
			charV := int(s[v] - 'a')
			for c := 0; c < 26; c++ {
				val := dp[u][c]
				if c == charV {
					val++
				}
				if val > dp[v][c] {
					dp[v][c] = val
				}
			}
			inDegree[v]--
			if inDegree[v] == 0 {
				queue = append(queue, v)
			}
		}
	}

	if processed < n {
		fmt.Println("-1")
	} else {
		fmt.Println(ans)
	}
}
```