package main

import . "fmt"
import . "os"
import bf "bufio"

func main() {
	rd:=bf.NewReader(Stdin)
	
	var n,m int
	Fscan(rd,&n,&m)
	graph := make([][]int, n+1)
	for i := 0; i < m; i++ {
		var a, b int
		Fscan(rd,&a,&b)
		graph[a] = append(graph[a], b)
		graph[b] = append(graph[b], a)
	}
	
	current := []int{1}
	visited := make([]bool, n+1)
	visited[1] = true
	cnt := 1
	for len(current) > 0 {
		tmp := []int{}
		for _, v := range current {
			for _, e := range graph[v] {
				if visited[e] {
					continue
				}
				if e == n {
					Println(cnt)
					return
				}
				visited[e] = true
				tmp =append(tmp, e)
			}
		}
		current = tmp
		cnt++
	}
	Println(-1)
}


/*
考察

入力の辺を全部つないでグラフにして
頂点1から頂点Nまでの最短ルートの辺の数が答え

*/