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

int main() {
    int N;
    ll a[1010] = {}, b[1010] = {};
    cin >> N;
    for (int i = 0; i < N; i++) {
        cin >> a[i] >> b[i];
    }

    bool parity[2] = {};
    ll SUM = 0, MAX = 0;
    for (int i = 0; i < N; i++) {
        parity[a[i] % 2] = true;
        MAX = max(MAX, a[i] + 4 * b[i]);
        SUM += a[i] + 4 * b[i];
    }
    ll ans = -1;
    if (parity[0] != parity[1]) {
        ans = (MAX * N - SUM) / 2;
    }

    cout << ans << endl;
}