#include<iostream>
using namespace std;

int main(){
  int n,m;
  cin>>n>>m;
  vector<vector<int>>data(2,vector<int>(m));
  for(int i=0;i<m;i++){
    cin>>data.at(i).at(0)>>data.at(i).at(1);
  }
  for(int i=0;i<m;i++){
    if(n=3&&data.at(i).at(0)==1&&data.at(i).at(0)==0){
      cout<<-1<<endl;
      return 0;
    }
    if(n=2&&data.at(i).at(0)==2&&data.at(i).at(0)==0){
      cout<<-1<<endl;
      return 0;
    }
    for(int j=0;j<m;j++){
      if(data.at(i).at(0)==data.at(j).at(0)&&data.at(i).at(1)!=data.at(j).at(1)){
        cout<<-1<<endl;
        return 0;
      }
    }
  }
}
  
  
  
    
    
    
    
    
 