/* -*- coding: utf-8 -*-
 *
 * 178.cc: No.178 美しいWhitespace (1) - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 1000;

/* typedef */

typedef long long ll;

/* global variables */

int abis[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  cin >> n;

  int maxab = 0;
  for (int i = 0; i < n; i++) {
    int a, b;
    cin >> a >> b;
    abis[i] = a + 4 * b;
    if (maxab < abis[i]) maxab = abis[i];
  }

  int p = (abis[0] & 1);
  for (int i = 1; i < n; i++)
    if ((abis[i] & 1) != p) {
      puts("-1");
      return 0;
    }

  ll sum = 0;
  for (int i = 0; i < n; i++)
    sum += (maxab - abis[i]) / 2;

  printf("%lld\n", sum);
  return 0;
}