#include <iostream>
#include <iomanip>
#include <vector>

typedef long long ll;

int main(void)
{
  std::cin.tie(0);
  std::ios::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(8);
  int n;
  std::cin >> n;
  ll w_max = 0, w_sum = 0;
  for (int i = 0; i < n; ++i) {
    ll a, b;
    std::cin >> a >> b;
    ll w_now = a + 4*b;
    if (i != 0 && w_now%2 != w_max%2) {std::cout << -1 << std::endl; return 0;}
    w_max = std::max(w_max, w_now);
    w_sum += w_now;
  }
  ll w_diff = w_max * n - w_sum;
  std::cout << w_diff/2 << std::endl;
  return 0;
}