public class No_185 {

	public static void main(String[] args) {
		java.util.Scanner sc = new java.util.Scanner(System.in);
		int n = sc.nextInt();
		int x = sc.nextInt();
		int y = sc.nextInt();
		int ans = y - x;

		for (int i = 1; i < n; i++) {
			x = sc.nextInt();
			y = sc.nextInt();
			if (ans != y - x) {
				ans = -1;
				break;
			}
		}

		if (ans >= 0) {
			System.out.println(ans);
		} else {
			System.out.println(-1);
		}
		sc.close();

	}

}