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 ans = 0;
			int tmp = 0;

			for (int i = 0; i < N; i++) {
				int x = sc.nextInt();
				int y = sc.nextInt();
				ans = y - x;

				if (i == 0) {
					tmp = ans;
				}
				if (ans < 1 || tmp != ans) {
					ans = -1;
					break;
				}
				tmp = ans;
			}
			System.out.println(ans);

		}
	}
}