#include <iostream>

int main() {
	int n;
	std::cin >> n;
	int temp;
	{int x, y;
	std::cin >> x >> y;
	temp = y - x; }
	for (auto i = 1; i < n; ++i) {
		int x, y;
		std::cin >> x >> y;
		if (temp != (y - x) || temp < 1) {
			std::cout << -1 << std::endl;
			return 0;
		}
	}
	if (temp > 0) {
		std::cout << temp << std::endl;
	}
	else {
		std::cout << -1 << std::endl;
	}
	return 0;
}