#define _USE_MATH_DEFIMES
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cctype>
#include <climits>
#include <clocale>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <regex>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

const int MOD = 1'000'000'007;
const int MOD2 = 998'244'353;
const int INF = 1'000'000'000; //1e9
const int NIL = -1;
const long long LINF = 1'000'000'000'000'000'000; // 1e18
const long double EPS = 1E-10L;

template<class T, class S> inline bool chmax(T &a, const S &b){
    if(a < b){a = b; return true;}
    return false;
}
template<class T, class S> inline bool chmin(T &a, const S &b){
    if(b < a){a = b; return true;}
    return false;
}
template<class T, class Container> inline bool exist(Container &s, const T &e){
    return (s.find(e) != std::end(s));
}
template<class T> inline bool inside(T x, T lx, T rx){
    return (std::clamp(x, lx, rx) == x);
}
template<class T> inline bool inside(T x, T y, T lx, T rx, T ly, T ry){
    return inside(x, lx, rx) && inside(y, ly, ry);
}








void dijkstraWithoutCost(int s, std::vector<std::vector<int>> &G, std::vector<long long> &d, std::vector<int> &prv){
    //pair<int, int> first: 距離　second: 頂点
    std::priority_queue<std::pair<long long, int>,
                        std::vector<std::pair<long long, int>>,
                        std::greater<std::pair<long long, int>>> que;
    int V{int(G.size())};
    d.resize(V, LINF+3);
    prv.resize(V, NIL);
    d[s] = 0;
    que.emplace(0, s);

    while(!que.empty()){
        std::pair<long long, int> p{que.top()}; que.pop();
        int v{p.second};
        if(d[v] < p.first) continue;
        for(int &u: G[v]){
            if(d[u] > d[v] + 1){
                d[u] = d[v] + 1;
                prv[u] = v;
                que.emplace(d[u], u);
            }
        }
    }
}

int main(){
    int N, M; std::cin >> N >> M;
    std::vector<std::vector<int>> G(N);
    {
        int a, b;
        for(int i{0}; i < M; ++i){
            std::cin >> a >> b; --a; --b;
            G[a].push_back(b);
            G[b].push_back(a);
        }
    }
    std::vector<long long> d;
    std::vector<int> prv;
    dijkstraWithoutCost(0, G, d, prv);
    std::cout << (d[N-1] <= N ? d[N-1] : -1) << std::endl;
    return 0;
}