package yukicoder;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int candidate = 0;
		for (int i = 0; i < N; i++) {
			int x = sc.nextInt();
			int y = sc.nextInt();
			if (i == 0) {
				candidate = y - x;
			}
			if (candidate <= 0 || candidate != y - x) {
				System.out.println(-1);
				return;
			}
		}
		System.out.println(candidate);

	}

}