package me.yukicoder.lv1;
import java.util.Scanner;

public class No0185 {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int N = sc.nextInt();
			int x = sc.nextInt();
			int y = sc.nextInt();
			int ans = y - x;

			for (int i = 0; i < N - 1; i++) {
				x = sc.nextInt();
				y = sc.nextInt();
				if (ans != y - x || ans < 0) {
					ans = -1;
					break;
				}
			}

			System.out.println(ans);
		}
	}
}