#include<bits/stdc++.h>
using namespace std;
using Int = long long;

template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  Int n;
  cin>>n;
  vector<Int> a(n),b(n);
  for(Int i=0;i<n;i++) cin>>a[i]>>b[i];
  Int x=a[0]+b[0]*4;
  for(Int i=0;i<n;i++){
    Int y=a[i]+b[i]*4;
    if((x+y)&1){
      cout<<-1<<endl;
      return 0;
    }
    chmax(x,y);
  }
  Int ans=0;
  for(Int i=0;i<n;i++){
    Int y=a[i]+b[i]*4;
    ans+=(x-y)/2;
  }
  cout<<ans<<endl;
  return 0;
}