#include <iostream>
using namespace std;
int main() {
	int N;
	cin >> N;
	long long X, Y,temp = -1;
	for (int i = 0; i < N;i++) {
		cin >> X >> Y;
		if (Y - X <= 0) { cout << -1 << endl; return 0; }
		if (temp == -1) { temp = Y - X; continue; }
		if (temp != Y - X) { cout << -1 << endl; return 0; }
	}
	cout << temp << endl;
	return 0;
}