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

int main(){
    int N;
    long initial;
    long a,b;
    long M=0;
    long s=0;
    cin>>N;
    for(int i=0;i<N;i++){
        cin>>a>>b;
        if(i==0){
            initial=(a+b*4);
        }
        M=max(M,a+b*4);
        s+=a+b*4;
        if((a+b*4)%2!=initial%2){
            cout<<-1<<endl;
            return 0;
        }
    }
    cout<<(M*N-s)/2<<endl;
}