#include <bits/stdc++.h> 

using namespace std;
typedef long long ll;

int main() {
    int N;
    cin >> N;
    vector<int> A(N), B(N);
    vector<ll> space(N);
    ll mv = -1;
    for (int i = 0; i < N; i++) {
        cin >> A[i] >> B[i];
        space[i] = A[i] + 4 * B[i];
        mv = max(mv, space[i]);
    }

    ll count = 0;
    for (int i = 0; i < N; i++) {
        if ((mv - space[i]) % 2 != 0) {
            count = -1;
            break;
        } else {
            count += (mv - space[i]) / 2;
        }
    }
    cout << count << endl;
}