#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

// g <- pair < v , cost > 
template < class T >
vector< T > dijkstra(vector<vector<pair<int, T>>> &graph, int s) {
    T INF = numeric_limits< T >::max();
    vector<T> dist(graph.size(), INF);
    priority_queue<pair<T,int>, vector<pair<T,int>>, greater<pair<T,int>>> q;
    q.push({dist[s] = T(0), s});
    while(!q.empty()){
        auto [uc, ui] = q.top(); q.pop();
        if(uc != dist[ui]) continue;
        for(auto [vi, vc] : graph[ui]) if(dist[vi] > uc + vc) 
            q.push({dist[vi] = uc + vc, vi});
    }
    return dist;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N,M; cin >> N >> M;
    vector<vector<pair<int,int>>> G(N);
    rep(i,M) {
        int a,b; cin >> a >> b; a--; b--;
        G[a].push_back({b, 1});
        G[b].push_back({a, 1});
    }

    int ans = dijkstra(G, 0)[N - 1];
    cout << (ans < int(1e9) ? ans : -1) << endl;
}