import java.util.Scanner;

public class Main {
     public static void main(String []args){
         Scanner in = new Scanner(System.in);
         int loops = in.nextInt();
         int x = in.nextInt(), y = in.nextInt();
         int ans = y - x;
         
         if(ans <= 0){
             System.out.println(-1);
             return;
         }
         
         while(in.hasNextInt()){
             x = in.nextInt();
             y = in.nextInt();
             if(ans != (y - x)){
                 System.out.println(-1);
                 return;
             }
         }
         System.out.println(ans);
     }
}