#include <bits/stdc++.h>

using namespace std;

int a[1000];

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

    int ma = 0;

    for (int i = 0; i < n; ++i) {
        int b, c; cin >> b >> c;
        a[i] = b + 4 * c;
        ma = max(ma, a[i]);
    }

    int ans = 0;

    for (int i = 0; i < n; ++i) {
        if ((ma - a[i]) & 1) {
            cout << -1 << endl;
            return 0;
        }
        ans += (ma - a[i]) / 2;
    }

    cout << ans << endl;
}