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

int main() {
    int n;
    cin >> n;
    vector<int> x;
    int m, f =-1;
    while(n--) {
        int a,b,c = 0;
        cin >> a>> b;
        c = a+4*b;
        x.push_back(c);
        if (f==-1){
            f = c % 2;
        }else if (f != c%2) {
            f = -1;
            break;
        }
        if (m < c) m = c;
    }
    if (f==-1) {
        cout<<-1<<endl;
        return 0;
    }
    int ans = 0;
    for (int i = 0; i < x.size(); ++i) {
        ans += (m - x[i])/2;
    }
    cout<<ans<<endl;
    return 0;
}