#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, m; cin >> n >> m;
	vector<vector<int>>to(n);
	rep(i, m) {
		int u, v; cin >> u >> v;
		u--, v--;
		to[u].push_back(v);
	}
	vector dp(n, vector<int>(4, INF));
	dp[0][0] = 0;
	std::queue<std::pair<int, int>> q;
	q.push({ 0,0 });
	while (!q.empty()) {
		int i = q.front().first;
		int c = q.front().second;
		q.pop();
		rep(j, to[i].size()) {
			int nc = c;
			if (n - 1 == to[i][j]) {
				nc |= 1;
			}
			if (n - 2 == to[i][j]) {
				nc |= 2;
			}if (chmin(dp[to[i][j]][nc], dp[i][c] + 1)) q.emplace(to[i][j], nc);
		}
	}
	int ans = dp[0][3];
	if (INF == ans)ans = -1;
	cout << ans << endl;
	return 0;
}