#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
int main() {
    // Input
    int N;
    cin >> N;
    vector<int> w(N);
    rep(i, N) {
        int a, b;
        cin >> a >> b;
        w[i] = a + 4 * b;
    }

    // Trivial Exception
    rep(i, N-1) {
        if (w[i] % 2 != w[i+1] % 2) {
            cout << -1 << endl;
            return 0;
        }
    }

    // Calculation
    int maxw = *max_element(w.begin(), w.end());
    long long ans = 0;
    rep(i, N) ans += (maxw - w[i]) / 2;

    // Output
    cout << ans << endl;
}