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

int main(void) {
    int n;
    cin >> n;
    vector<ll> a(n),b(n);
    ll l = 0;
    for(int i=0;i<n;i++){
        cin >> a[i] >> b[i];
        l = max(a[i]+b[i]*4,l);
    }

    ll sum = 0;
    for(int i=0;i<n;i++){
        if( (l -  a[i]-b[i]*4) % 2 != 0){
            cout << -1 << endl;
            return 0;
        }
        sum += (l - a[i] - b[i] * 4)/2;
    }
    cout << sum << endl;

    return 0;
}