package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	scanner := bufio.NewScanner(os.Stdin)
	scanner.Split(bufio.ScanWords)
	
	scanInt := func() int {
		scanner.Scan()
		res := 0
		for _, b := range scanner.Bytes() {
			res = res*10 + int(b-'0')
		}
		return res
	}

	if !scanner.Scan() {
		return
	}
	
	n := 0
	for _, b := range scanner.Bytes() {
		n = n*10 + int(b-'0')
	}
	m := scanInt()

	adj := make([][]int, n+1)
	for i := 0; i < m; i++ {
		u := scanInt()
		v := scanInt()
		adj[u] = append(adj[u], v)
		adj[v] = append(adj[v], u)
	}

	color := make([]int, n+1)
	ans := 0

	for i := 1; i <= n; i++ {
		if color[i] == 0 {
			comp := []int{}
			q := []int{i}
			color[i] = 1
			possible := true

			for head := 0; head < len(q); head++ {
				u := q[head]
				comp = append(comp, u)
				for _, v := range adj[u] {
					if color[v] == 0 {
						color[v] = 3 - color[u]
						q = append(q, v)
					} else if color[v] == color[u] {
						possible = false
					}
				}
			}

			if !possible {
				fmt.Println("-1")
				return
			}

			maxD := 0
			dist := make([]int, n+1)
			
			for _, start := range comp {
				for _, v := range comp {
					dist[v] = -1
				}
				dist[start] = 0
				qBfs := []int{start}
				currentMax := 0

				for head := 0; head < len(qBfs); head++ {
					u := qBfs[head]
					currentMax = dist[u]
					for _, v := range adj[u] {
						if dist[v] == -1 {
							dist[v] = dist[u] + 1
							qBfs = append(qBfs, v)
						}
					}
				}
				if currentMax > maxD {
					maxD = currentMax
				}
			}
			ans += maxD
		}
	}
	fmt.Println(ans)
}