#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)

const double PI = acos(-1);

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n;
  cin >> n;
  vector<int> x(n);
  REP (i, n) {
    int a, b;
    cin >> a >> b;
    x[i] = a + 4 * b;
  }

  int y = *max_element(x.begin(), x.end());
  long long sum = 0;
  REP (i, x.size()) sum += y - x[i];
  if (sum % 2)
    cout << -1 << endl;
  else
    cout << sum / 2 << endl;
    
  return 0;
}