#include "bits/stdc++.h"
using namespace std;

int main() {
	int N, X, Y, ans;
	bool check = true;
	cin >> N >> X >> Y;
	ans = Y - X;
	if (ans <= 0) check = false;
	for (int i = 1; i < N; i++) {
		cin >> X >> Y;
		if (X >= Y) check = false;
		else if (Y - X != ans) check = false;
	}
	if (check) cout << ans;
	else cout << -1;
}