#include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
long long int a[1000],b[1000],ki,gu,ma,need,cop;
long long int all[1000];
int main(){
    int N,cou;
    cin>>N;
    for(int i=0;i<N;i++){
        cin>>a[i]>>b[i];
        all[i]=a[i]+b[i]*4;
        if(all[i]%2==0)gu++;
        else ki++;
        ma=max(ma,all[i]);
    }
    cop=ma;
    if(gu==N||ki==N){
        for(int i=0;i<N;i++){
            need+=ma-all[i];
        }
        cout<<need/2<<endl;
    }
    else cout<<-1<<endl;
}