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

#define LL long long
#define rep(i, a, b) for(LL i=(a); i<(b); i++)
template <typename T> bool chmin(T& a, const T& b) { return a>b ? a=b,1 : 0; }

int main() {
	int N, M; cin >> N >> M;
	vector<int> E[1<<17];
	rep(_, 0, M) {
		int u, v; cin >> u >> v; u--; v--;
		E[u].push_back(v);
		E[v].push_back(u);
	}
        
    const int inf = 1<<27;
    deque<pair<int, int>> que = {{0, 0}};
    int d[4][1<<17]; rep(i, 0, 4) rep(j, 0, 1<<17) { d[i][j] = inf; }
    d[0][0] = 0;
    
    while(!que.empty()) {
        auto [k, i] = que.front(); que.pop_front();
        
        for(auto j: E[i]) {
        	int xk = k;
        	if (j == N-2) { xk |= 1; }
        	if (j == N-1) { xk |= 2; }
        	
            if (chmin(d[xk][j], d[k][i]+1)) {
                que.push_back(make_pair(xk, j));
            }
        }
    }
    
    
    if (d[3][0] == inf) { d[3][0] = -1; }
    cout << d[3][0];
}