#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<iomanip>
#include<cstring>
#include<map>
#include<vector>
using namespace std;
typedef long long int ll;

int main(){

  ll n, a[1000], b[1000], total=0, max;
  cin >> n;
  for(int i=0; i<n; i++){
    cin >> a[i] >> b[i];
    if(a[i]+4*b[i]>total){
      total=a[i]+4*b[i];
    }
  }
  max=total;
  total=0;
  for(int i=0; i<n; i++){
    total+=max-(a[i]+4*b[i]);
    if(total%2!=0){
      cout << -1 << endl;
      return 0;
    }
  }

  cout << total/2 << endl;

  return 0;
}