#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001

int main(){
	
	int n,m;
	cin>>n>>m;
	
	mcf_graph<int,int> G(n);
	
	rep(i,m){
		int a,b;
		cin>>a>>b;
		a--;b--;
		G.add_edge(a,b,1,1);
		G.add_edge(b,a,1,1);
	}
	
	auto ret = G.flow(0,n-1,1);
	
	if(ret.first==0)cout<<-1<<endl;
	else cout<<ret.second<<endl;
	
    return 0;
}