package yukicoder;

import java.util.Scanner;

public class Yuki185 {
	
	public static void main(String[] args) {
		
		Scanner scan = new Scanner(System.in);
		int num = scan.nextInt();
		int[][] num_arr = new int[num][2];
		
		for(int i = 0; i < num ; i++) {
			for(int j = 0; j < 2; j++) {
				num_arr[i][j] = scan.nextInt();
			}
		}
		scan.close();
		int dif_numbers = 0;
		int parse_numbers = -1;
		for(int i = 0; i < num ; i++) {
			dif_numbers = num_arr[i][1] - num_arr[i][0];
			
			if(dif_numbers < 0 ) {
				break;
			} else {
				if(i > 0 && parse_numbers != dif_numbers){
					parse_numbers = -1;
					break;
				}
				parse_numbers = dif_numbers;
			}
		}
		System.out.println(parse_numbers);
		scan.close();	
	}
}