package main
 
import (
	"bufio"
	"fmt"
	"os"
)
 
const INF = int(1e9)
 
func main() {
	in := bufio.NewReader(os.Stdin)
 
	var n, m int
	if _, err := fmt.Fscan(in, &n, &m); err != nil {
		// вообще нет входа
		fmt.Print(-1)
		return
	}
 
	g := make([][]int, n)
 
	// Читаем ДО m рёбер, но если ввод закончился — не зависаем
	for i := 0; i < m; i++ {
		var a, b int
		if _, err := fmt.Fscan(in, &a, &b); err != nil {
			break // EOF или другой конец ввода
		}
		a--
		b--
		if a >= 0 && a < n && b >= 0 && b < n && a != b {
			g[a] = append(g[a], b)
			g[b] = append(g[b], a)
		}
	}
 
	best := INF
 
	dist := make([]int, n)
	parent := make([]int, n)
	q := make([]int, n)
 
	for s := 0; s < n; s++ {
		for i := 0; i < n; i++ {
			dist[i] = -1
			parent[i] = -1
		}
 
		head, tail := 0, 0
		dist[s] = 0
		q[tail] = s
		tail++
 
		for head < tail {
			v := q[head]
			head++
 
			if dist[v]*2+1 >= best {
				continue
			}
 
			for _, to := range g[v] {
				if dist[to] == -1 {
					dist[to] = dist[v] + 1
					parent[to] = v
					q[tail] = to
					tail++
				} else if parent[v] != to {
					cycleLen := dist[v] + dist[to] + 1
					if cycleLen < best {
						best = cycleLen
					}
				}
			}
		}
	}
 
	if best == INF {
		fmt.Print(-1)
	} else {
		fmt.Print(best)
	}
}
 
 
 
 

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