#include <bits/stdc++.h>
using namespace std;
#define MOD 1000000007
#define MOD2 998244353
#define rep(i,n) for (int i = 0; i < (int)(n); i++)
#define reps(i,s,n) for (int i = (int)(s); i < (int)(n); i++)
#define repit(i,C) for (auto i = (C).begin(); i != (C).end(); i++)
#define pr(a) cout << a
#define prl(a) cout << (a) << endl
#define prld(a) cout << setprecision(15)<< (a) << endl 
#define allrange(a) a.begin(),a.end()

template<typename T = int>
struct Edge {
  int src, dst;
  T weight;
  Edge(int src, int dst, T weight) :
    src(src), dst(dst), weight(weight) { };
  Edge() {};
};
template<typename T = int>
bool operator < (const Edge<T> &e, const Edge<T> &f) {
  return e.weight != f.weight ? e.weight < f.weight : // コストが低いほう優先
    e.src != f.src ? e.src < f.src : e.dst < f.dst;
};

template <typename T= int>
using Edges = std::vector<Edge<T>>;
using Vertexs = std::vector<int>;


template<typename T = int>
struct Graph { //directed graph
    Vertexs vertexs;
    map<int,int> vindex;
    Edges<T> edges;
    std::vector<
      std::vector< 
       std::pair<T,int>
    >> adjacent_to, adjacent_from;

    Graph(): vertexs(Vertexs (0)),edges(Edges<T>(0)),vindex(),
      adjacent_to(std::vector<std::vector<std::pair<T,int>>>(0)),
      adjacent_from(std::vector<std::vector<std::pair<T,int>>>(0)) {};

    void addv(int t){
        vertexs.push_back(t);
        vindex[t]=vertexs.size()-1;
        adjacent_to.push_back(std::vector<std::pair<T,int>>(0));
        adjacent_from.push_back(std::vector<std::pair<T,int>>(0));
    }
    void adde(int s,int d,T w){
        edges.push_back(Edge<T>(s,d,w));
        adjacent_to.at(vindex[s]).push_back(std::make_pair(w,d));
        adjacent_from.at(vindex[s]).push_back(std::make_pair(w,d));
    }

};

template<typename T = int>
struct UndirectedGraph : Graph<T>{ //undirected graph
    void adde(int s,int d,T w){
        this.adde(s,d,w);
        this.adjacent_to.at(d).push_back(std::make_pair(w,s));
        this.adjacent_from.at(d).push_back(std::make_pair(w,s));
    }
};

/* 2021/06/20

 unordered_setを使って拡張可能なデータ構造。
  find　　所属判定
  insert  元の挿入
 が平均o(1)で可能。
  グループの合併は再帰関数を使用
  
  元の削除はできない
*/
vector<int> graph_bfs(struct Graph<int>& G,int s=0)
{
  int N = G.vertexs.size();
  vector<int> out(N);
  vector<bool> visited(N);
  queue<int> work;

  for(int i=0;i<N;i++)
  {
    out[i]=INT_MAX;
    visited[i]=false;
  }
  out[s]=0;
  work.push(s);

  while(!work.empty())
  {
    int pt = work.front();
    work.pop();
    for(auto e:G.adjacent_from[pt])
      if(out[e.second]==INT_MAX) out[e.second]=out[pt]+e.first;
      else out[e.second] = min(out[e.second],out[pt]+e.first);
    
    if(visited[pt]) continue;
     visited[pt]=true;
    for(auto e:G.adjacent_from[pt])
       if(!visited[e.second]) work.push(e.second);
   }

   for(int &l:out) if(l==INT_MAX) l=-1;
   return out;
};


int main(){
   //1変数入力
    int N,M;
    cin >> N >> M;
    Graph<int> G;

    rep(i,N) G.addv(i); 

    rep(i,M){
        int u,v;
        cin >> u >> v;
        u--;v--;
        G.adde(u,v,1);
        G.adde(v,u,1);
    }

    auto ans =  graph_bfs(G,0);
    prl(ans[N-1]);

}