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

int main(){
    int N,a,b;
    long long ans=0;
    vector<int> v;

    cin >> N;
    while(cin >> a >> b) v.push_back(a+b*4);
    sort(v.begin(),v.end());

    for(int i=0;i<N-1;i++){
        if((v.back()-v[i])%2!=0){
            cout << -1 << endl;
            return 0;
        }
        ans+=(v.back()-v[i])/2;
    }
    cout << ans << endl;

    return 0;
}