#include <iostream>
#include <algorithm>
using namespace std;
int t[1000];

int main() {
  int n, a, b, f, m = 0;
  long long cnt = 0;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a >> b;
    t[i] = a + 4 * b;
    if (i == 0) f = t[i] % 2;
    if (f != t[i] % 2) {
      cout << -1 << endl;
      return 0;
    }
    m = max(t[i], m);
  }
  for (int i = 0; i < n; i++) {
    cnt += m - t[i];
  }
  cout << cnt / 2 << endl;
  return 0;
}