#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];
    if(i!=0){
      if(a[i]%2 != a[i-1]%2){
        cout<<-1<<endl;
        return 0;
      }
    }
    upr = max( upr, a[i]+4 * b[i] );
  }
  int cnt = 0;
  for (int i = 0; i< n; i++){
    cnt += ( upr - (a[i]+4*b[i]) )/2;
  }
  cout<<cnt<<endl;
  return 0;
}