#include <iostream>
#include <cmath>
using namespace std;

int main(){
   int ans = 0,lenmax = 0,n;
   bool oe;
   cin >> n;
   int a[n],b[n],length[n];
   for(int i = 0;i < n;i++){
      cin >> a[i] >> b[i];
      length[i] = a[i] + 4 * b[i];
      if(!i) oe = length[i] % 2;
      else if(oe != length[i] % 2){
         cout << -1 << endl;
         return 0;
      }
      lenmax = max(lenmax,length[i]);
   }
   for(int i = 0;i < n;i++) ans += lenmax - length[i];
   cout << ans / 2 << endl;
}