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

int main() {
    int n;
    cin >> n;
    ll sum = 0;
    int p;
    ll mx = 0;
    for (int i = 0; i < n; i++) {
        ll a, b;
        cin >> a >> b;
        if (i == 0) p = a%2;
        else if (a%2 != p) {
            puts("-1");
            return 0;
        }
        sum += a+4*b;
        mx = max(mx, a+4*b);
    }
    cout << (mx*n - sum) / 2 << endl;
    return 0;
}