#include<bits/stdc++.h>
using namespace std;

int main(){
  int n;
  cin >> n;
  vector<int> space(n);
  int a, b;
  for(int i = 0; i < n; i++){
    cin >> a >> b;
    space[i] = a + (4 * b);
  }
  
  sort(space.begin(), space.end());
  reverse(space.begin(), space.end());
  
  int max_space = space[0];
  long long count = 0;
  for(int i = 1; i < n; i++){
    int a = max_space - space[i];
    if(a % 2 != 0){
      count = -1;
      break;
    }else{
      count += a / 2;
    }
  }
  cout << count << endl;
}