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

int main() {
	int n;

	std::cin >> n;

	std::vector<int> space(n, 0), tab(n, 0);

	for (int i = 0; i < n; i++) {
		std::cin >> space[i] >> tab[i];
	}

	std::vector<int> width(n, 0);
	for (int i = 0; i < n; i++) {
		width[i] = space[i] + 4 * tab[i];
	}

	for (int i = 0; i < n; i++) {
		if (width[i] % 2 != width[0] % 2) {
			std::cout << -1 << std::endl;
			return 0;
		}
	}

	int max = *std::max_element(width.begin(), width.end());

	int count = 0;
	for (int i = 0; i < n; i++) {
		count += (max - width[i]);
	}

	std::cout << count / 2 << std::endl;

	return 0;
}