#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

bool func(const vector<int>& vec) {
    vector<int> evenOrOdd;
    for (size_t i = 0; i < vec.size(); ++i) {
        if (vec.at(i) % 2 == 0) {
            evenOrOdd.push_back(0); //even
        } else {
            evenOrOdd.push_back(1); //odd
        }
    }
    bool flag = true; //全部evenか全部oddならtrue, 要素ひとつでも違えばfalse
    for (size_t j = 0; j < (vec.size() - 1); ++j) {
        if (evenOrOdd.at(j) != evenOrOdd.at(j+1)) {
            flag = false;
        }
    }
    return flag;
}

int main() {
    int n, a, b;
    cin >> n;
    vector<int> wa;
    for (int i = 0; i < n; ++i) {
        cin >> a >> b;
        wa.push_back(a + 4 * b);
    }
    if (func(wa) == true) {
        int p = *max_element(wa.begin(), wa.end());
        int ans = 0;
        for (int j = 0; j < n; ++j) {
            ans += (p - wa.at(j)) / 2;
        }
        cout << ans << endl;
    } else {
        cout << -1 << endl;
    }
    return 0;
}