#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
#define rep(i,n) for(int i=0;i<n;i++)
#define int long long

signed main() {
  int n;
  cin>>n;
  int a[n];
  int b[n];
  int sum=0;
  int maxab=0;
  rep(i,n){
    cin>>a[i]>>b[i];
    maxab=max(maxab,a[i]+b[i]*4);
  }
  rep(i,n){
    if((maxab-a[i]-b[i]*4)%2==1){
      cout<<-1<<endl;
      return 0;
    }
    sum+=(maxab-a[i]-b[i]*4)/2;
  }
  cout<<sum<<endl;
  return 0;
}