#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

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

  ll maxs = 0;
  vector<ll> a(n), b(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i] >> b[i];
    if (a[i] + 4 * b[i] <= maxs) continue;
    maxs = a[i] + 4 * b[i];
  }

  ll ans = 0;
  for (int i = 0; i < n; i++) {
    if (ans < 0) continue;
    ll diff = maxs - a[i] - 4 * b[i];
    if (diff % 2 == 0) ans += diff / 2;
    else ans = -1;
  }
  cout << ans << endl;

  return 0;
}