#include <bits/stdc++.h>

using namespace std;

int main()
{
  long N, ret = 0, all = 0;
  bool diff[2] = {};
  long A[1000], B[1000];

  cin >> N;
  for(int i = 0; i < N; i++) {
    cin >> A[i] >> B[i];
    long space = A[i] + 4 * B[i];
    diff[space & 1] = true;
    ret = max(ret, space);
    all += space;
  }
  if(diff[0] & diff[1]) cout << -1 << endl;
  else cout << (ret * N - all) / 2 << endl;
}