#ifdef SENJAN
#include <ku/debug.hpp>
#else
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#define debug(...)
#endif
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll=long long;
using ull=unsigned long long;
using ld=long double;
using graph=vector<vector<int>>;
template<class T> using wgraph=vector<vector<pair<int,T>>>;
template<class T> using min_priority_queue=priority_queue<T,vector<T>,greater<T>>;
constexpr int INF32=INT_MAX/2;
constexpr ll INF64=1LL<<60;
constexpr ld PI=3.14159265358979323846;
constexpr int dx[]={0,0,-1,1,-1,-1,1,1},dy[]={-1,1,0,0,-1,1,-1,1};
#define rep(i,s,n) for(ll i=(ll)(s);i<(ll)(n);i++)
#define rrep(i,s,n) for(ll i=(ll)(s);i>=(ll)(n);i--)
template<class T> inline bool chmax(T &a,T b){return a<b?a=b,true:false;}
template<class T> inline bool chmin(T &a,T b){return a>b?a=b,true:false;}
void _main();
int main(){cin.tie(0)->sync_with_stdio(0);cout<<fixed<<setprecision(16);_main();}

void _main(){
	int N, M; cin >> N >> M;
	graph G(N);
	rep(i,0,M){
		int u, v; cin >> u >> v;
		u--, v--;
		G[u].push_back(v);
	}

	auto bfs = [&](int s, vector<ll> &dist) -> void{
		dist.resize(N, INF64);
		dist[s] = 0;
		queue<int> que;
		que.push(s);
		while(!que.empty()){
			int v = que.front();
			que.pop();
			for(int nv:G[v]) if(dist[nv]==INF64){
				dist[nv] = dist[v]+1;
				que.push(nv);
			}
		}
	};

	vector<ll> dist_a, dist_b, dist_c;
	int a = 0, b = N-2, c = N-1;
	bfs(a, dist_a);
	bfs(b, dist_b);
	bfs(c, dist_c);

	ll ans = INF64;
	// a->b->c->a
	chmin(ans, dist_a[b] + dist_b[c] + dist_c[a]);
	// a->c->b->a
	chmin(ans, dist_a[c] + dist_c[b] + dist_b[a]);
	// a->b->c->b->a
	chmin(ans, dist_a[b] + dist_b[c] + dist_c[b] + dist_b[a]);
	// a->c->b->c->a
	chmin(ans, dist_a[c] + dist_c[b] + dist_b[c] + dist_c[a]);
	// a->b->a->c->a
	chmin(ans, dist_a[b] + dist_b[a] + dist_a[c] + dist_c[a]);

	if(ans==INF64) cout << -1 << endl;
	else cout << ans << endl;
}