#include<bits/stdc++.h>
using namespace std;
int main(){
  long n,a,b;cin>>n;long mx=0;
  vector<long> w(n);
  for(long i=0;i<n;i++){
    cin>>a>>b;w[i]=a+4*b;
    mx=max(mx,w[i]);
  }
  long a1=0;
  long k1=0;
  long a2=0;
  long k2=0;
  for(long i=0;i<n;i++){
    if((mx-w[i])%2==1){
      k1=-1;
      a2+=(mx+1-w[i])/2;
    }else{
      a1+=(mx-w[i])/2;
      k2=-1;
    }
  }
  if(k1==-1&&k2==-1)cout<<-1<<endl;
  else if(k1==-1)cout<<a2<<endl;
  else if(k2==-1)cout<<a1<<endl;
}