#include <iostream>
using namespace std;

int main() {
    int n;
    long long mx = 0, ans = 0;
    cin >> n;
    long long spaces[n];
    for (int i = 0; i < n; i++){
        int a, b;
        cin >> a >> b;
        spaces[i] = a + b * 4;
        mx = max(mx,spaces[i]);
    }
    for (int i = 0; i < n; i++){
        if ((mx - spaces[i]) % 2 == 0)
            ans += (mx - spaces[i]) / 2;
        else {
            ans = -1;
            break;
        }
    }
    cout << ans << endl;
    return 0;
}