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

using lint = long long;

void solve() {
    int n;
    std::cin >> n;

    std::vector<lint> xs(n);
    for (auto& x : xs) {
        lint a, b;
        std::cin >> a >> b;
        x = a + b * 4;
    }

    std::sort(xs.begin(), xs.end());
    lint max = xs.back(), ans = 0;

    for (auto x : xs) {
        if ((max - x) % 2 != 0) {
            std::cout << -1 << std::endl;
            return;
        }

        ans += (max - x) / 2;
    }

    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}