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

int main() {
	int num, x, y;

	std::cin >> num;

	std::vector<int> ans;
	for (int i = 0; i < num; i++) {
		std::cin >> x >> y;
		ans.push_back(y - x);
	}
	
	if (std::count(ans.begin(), ans.end(), ans[0]) != ans.size()) {
		std::cout << -1 << std::endl;
	} else if(ans[0] < 0){
		std::cout << -1 << std::endl;
	} else {
		std::cout << ans[0] << std::endl;
	}

	return 0;
}