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