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

int main() {
  int N; cin >> N;
  vector<int64_t> vec(N);
  for (int i = 0; i < N; i++) {
    int64_t a, b;
    cin >> a >> b;
    vec[i] = a + 4 * b;
  }
  int64_t h = *max_element(vec.begin(), vec.end());
  int64_t ans = 0;
  for (auto &p : vec) {
    int64_t d = h - p;
    if (d & 1) {
      ans = -1;
      break;
    }
    ans += d / 2;
  }
  cout << ans << endl;
  return 0;
}