#include <bits/stdc++.h>
using namespace std;
int n,a[1010],maxn;
long long ans;
int main(){
    cin>>n;
    for(int i=0,x,y;i<n;++i){
        cin>>x>>y;
        a[i]=x+4*y;
        maxn=max(maxn,a[i]);
    }
    for(int i=0;i<n;++i){
        if(maxn%2!=a[i]%2){
            cout<<-1<<endl;
            return 0;
        }
        ans+=maxn-a[i];
    }
    cout<<ans/2<<endl;
    return 0;
}