#include <bits/stdc++.h>
using namespace std;
int main(){
  int N, M;
  cin >> N >> M;
  vector<vector<int>> E1(N), E2(N);
  vector<vector<int>> E(N);
  for (int i = 0; i < M; i++){
    int A, B;
    cin >> A >> B;
    A--;
    B--;
    E1[A].push_back(B);
    E2[B].push_back(A);
    E[A].push_back(B);
    E[B].push_back(A);
  }
  vector<bool> used1(N, false);
  used1[0] = true;
  queue<int> Q1;
  Q1.push(0);
  while (!Q1.empty()){
    int v = Q1.front();
    Q1.pop();
    for (int w : E1[v]){
      if (!used1[w]){
        used1[w] = true;
        Q1.push(w);
      }
    }
  }
  vector<bool> used2(N, false);
  used2[0] = true;
  queue<int> Q2;
  Q2.push(0);
  while (!Q2.empty()){
    int v = Q2.front();
    Q2.pop();
    for (int w : E2[v]){
      if (!used2[w]){
        used2[w] = true;
        Q2.push(w);
      }
    }
  }
  bool ok = false;
  vector<int> c(N, -1);
  c[0] = 0;
  queue<int> Q;
  Q.push(0);
  while (!Q.empty()){
    int v = Q.front();
    Q.pop();
    for (int w : E[v]){
      if (c[w] == -1){
        c[w] = 1 - c[v];
        Q.push(w);
      } else if (c[v] == c[w]){
        ok = true;
      }
    }
  }
  for (int i = 0; i < N; i++){
    if (!used1[i]){
      ok = false;
    }
  }
  for (int i = 0; i < N; i++){
    if (!used2[i]){
      ok = false;
    }
  }
  if (M <= N){
    ok = false;
  }
  if (!ok){
    cout << -1 << endl;
  } else {
    assert(false);
  }
}