#include <bits/stdc++.h>

using namespace std;

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

  cin >> N;
  while(N--) {
    long a, b;
    cin >> a >> b;
    long space = a + 4 * b;
    diff[space & 1] = true;
    ret = max(ret, space);
  }
  if(diff[0] & diff[1]) cout << -1 << endl;
  else cout << ret << endl;
}