#include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;
int main() {
  long long n,a,b,ans=0,M=0,cnt=0,s[1000]={},amari;
  cin >> n;
  for(int i=0; i<n; i++) {
      cin >> a >> b;
      s[i]=a+4*b;
      M=max(M,s[i]);
  }
  amari=s[0]%2;
  for(int i=0; i<n; i++) {
      if(s[i]%2!=amari) {
          cout << -1 << endl;
          return 0;
      }
      else {
          ans+=(M-s[i])/2;
      }
  }
  cout << ans << endl;
}