#include <iostream>
using namespace std;
int main(){
    long N,a,b,tab=0,sum=0;
    bool even=false,odd=false;
    cin >> N;
    for(int i=0;i<N;i++){
        cin >> a >> b;
        sum+=a+4*b;
        if((a+4*b)%2==0)even=true;
        else odd=true;
        tab=max(tab,a+4*b);
    }
    if(even!=odd)cout << (tab*N-sum)/2 << endl;
    else cout << "-1" << endl;
    return 0;
}