#include<iostream>
using namespace std;
int main(void){
  int n;
  int a[1001];
  int b[1001];
  int upr = -1;
  cin>>n;
  for( int i = 0;i < n; i++){
    cin>>a[i]>>b[i];
    upr = max( upr, a[i]+4 * b[i] );
  }
  bool odd = upr & 1;
  int cnt = 0;
  for (int i = 0; i< n; i++){
    if( odd ^ ((a[i]+4*b[i])&1)){
      cout<<-1<<endl;
      return 0;
    }
    cnt += ( upr - (a[i]+4*b[i]) )/2;
  }
  cout<<cnt<<endl;
  return 0;
}