/*||>>>> Praveen Ojha <<<<>>>> 18 June 2019 <<<<>>>> 10:54:18 <<<<||*/
// #pragma GCC optimize("Ofast") //(Very Fast but Inaccurate)
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native") //(Small operations in many loops)
// #pragma GCC optimize("unroll-loops")
// #pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define F first
#define S second
#define mod 1000000007
#define inf 1e18+5
#define sz(x) (int)x.size()
#define PI 3.141592653589793238510
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define __ ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define vi vector<int>
#define vpii vector<pair<int,int> > 
#define vvi vector<vector<int> >
#define PRINT_TIME cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s." <<endl;
#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename   enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i)); }
sim, class b dor(pair < b, c > d) {
  ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
  *this << "[";
  for (auto it = d.b; it != d.e; ++it)
    *this << ", " + 2 * (it == d.b) << *it;
  ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
typedef long double ld;
typedef pair<int,int> pii;
//Read Problems Carefully & Check for corner cases N=0,1 ?
///////////////////////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////////////////
const int N=500;
vi rail[N],road[N];
int vis[N];
int rr[N][N];
int cnt,n,m;

int bfs1(int uu){	
queue <pii> qq;
	qq.push({uu,0});
	while(!qq.empty()){
		pii u=qq.front();
		qq.pop();
		vis[u.F]=1;
		if(u.F==n)
			return u.S;
		for(int v:rail[u.F]){
			if(!vis[v])
				qq.push({v,u.S+1});
		}
	}
	return -1;
}
int bfs2(int uu){
	queue <pii> qq;
	qq.push({uu,0});
	while(!qq.empty()){
		pii u=qq.front();
		qq.pop();
		// debug()<< imie(u);
		vis[u.F]=1;
		if(u.F==n)
			return u.S;
		for(int v:road[u.F]){
			if(!vis[v])
				qq.push({v,u.S+1});
		}
	}
	return -1;
}
int32_t main(){__
	int x,y;
	cin>>n>>m;
	for(int i=1;i<=m;i++){
		cin>>x>>y;
		rr[x][y]=rr[y][x]=1;
		rail[x].push_back(y);
		rail[y].push_back(x);
	}
	for(int i=1;i<=n;i++){
		for(int j=i+1;j<=n;j++){
			if(!rr[i][j]){
				road[i].push_back(j);
				road[j].push_back(i);
			}
		}
	}
	if(rr[1][n]){
		cnt=bfs2(1);
			cout<<cnt<<"\n";
	}
	else{
		cnt=bfs1(1);
			cout<<cnt<<"\n";
	}
return 0;
}