#include <bits/stdc++.h>

using namespace std;

int main() {
    int n;
    cin >> n;

    vector<long long> vt(n);
    for (int i = 0; i < n; ++i) {
        long long a, b;
        cin >> a >> b;
        vt[i] = a + 4 * b;
    }

    long long wsMax = *std::max_element(vt.begin(), vt.end());
    long long ans = 0;
    for (int i = 0; i < n; ++i) {
        if ((wsMax - vt[i]) % 2 != 0) {
            cout << -1 << endl;
            return 0;
        } else {
            ans += (wsMax - vt[i]) / 2;
        }
    }

    cout << ans << endl;
    return 0;
}