#include<bits/stdc++.h>
using namespace std;

//* ATCODER
#include<atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
//*/

/* BOOST MULTIPRECISION
#include<boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;
//*/

typedef long long ll;

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)

template <typename T> bool chmin(T &a, const T &b) {
	if (a <= b) return false;
	a = b;
	return true;
}

template <typename T> bool chmax(T &a, const T &b) {
	if (a >= b) return false;
	a = b;
	return true;
}

template <typename T> T max(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmax(ret, a[i]);
	return ret;
}

template <typename T> T min(vector<T> &a){
	assert(!a.empty());
	T ret = a[0];
	for (int i=0; i<(int)a.size(); i++) chmin(ret, a[i]);
	return ret;
}

template <typename T> T sum(vector<T> &a){
	T ret = 0;
	for (int i=0; i<(int)a.size(); i++) ret += a[i];
	return ret;
}

int main(){
	int n, m; cin >> n >> m;
	vector ikeru(n, vector<int>(0));
	rep(i,0,m){
		int u, v; cin >> u >> v;
		u--; v--;
		ikeru[u].push_back(v);
	}
	
	vector dep(n, vector<int>(4));
	vector tansaku(n, vector<bool>(4));
	deque<pair<int,int>> mada = {pair(0, 0)};
	tansaku[0][0] = 1;
	while(!mada.empty()){
		auto [i,dat] = mada.front();
		mada.pop_front();
		for (int j:ikeru[i]){
			int tmp = dat;
			if (j==n-2) tmp |= 1;
			if (j==n-1) tmp |= 2;
			if (tansaku[j][tmp]) continue;
			tansaku[j][tmp] = 1;
			mada.push_back(pair(j,tmp));
			dep[j][tmp] = dep[i][dat] + 1;
		}
	}
	if (!tansaku[0][3]) cout << -1 << '\n';
	else cout << dep[0][3] << '\n';

}