#include <iostream>

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