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

int main() {
  int N;
  cin >> N;
  vector<int> V(N);
  int ma = 0;
  for (int i = 0, a, b, c = -1; cin >> a >> b; i++) {
    V.at(i) = a + b * 4, ma = max(ma, V.at(i));
    if (c == -1) c = V.at(i) % 2;
    else if (V.at(i) % 2 != c) return cout << -1 << "\n", 0;
  }
  long ans = 0;
  for (int i = 0; i < N; i++) ans += (ma - V.at(i)) / 2;
  cout << ans << "\n";
}