#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<map>
#include<queue>
#include<string>
#include<sstream>
#include<cmath>
using namespace std;

int main(){

  int n;
  cin >> n;
  int a, b;
  vector<long long int> w(n);
  for(int i=0; i<n; i++){
    cin >> a >> b;
    w[i] = a + 4 * b;
  }
  int mx = *max_element(w.begin(), w.end());
  int f = w[0]%2;
  long long int cnt = 0;
  for(int i=0; i<n; i++){
    if( w[i]%2 != f ){
      cout << -1 << endl;
      return 0;
    }
    cnt += (mx - w[i])/2;
  }
  cout << cnt << endl;


  return 0;
}