#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

int n;
ll a, b, step[1010];

int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a >> b;
    step[i] = a + 4 * b;
  }
  ll ma = *max_element(step, step + n);
  ll sum = 0;
  for (int i = 0; i < n; i++) {
    if ((ma - step[i]) % 2 == 1) {
      cout << -1 << endl;
      return 0;
    }
    sum += (ma - step[i]) / 2;
  }
  cout << sum << endl;
  return 0;
}