#include <iostream>

using namespace std;

int main() {
    int n;
    cin >> n;
    int length[n];
    int maxLength = 0;
    int sum = 0;
    for (int i = 0; i < n; i++) {
        int space, tab;
        cin >> space >> tab;
        length[i] = space + 4 * tab;
        if (i > 0 && ((length[i - 1] ^ length[i]) & 1) == 1) {
            cout << -1 << endl;
            return 0;
        }
        maxLength = max(maxLength, length[i]);
        sum += length[i];
    }
    cout << (maxLength * n - sum) / 2 << endl;
    return 0;
}