import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution{
   public static void lonelyinteger(int[] a) {
       //this method prints the lone integer
       for(int i: a){
           if(!copy(a, i)) System.out.println(i);
       }
    }

   public static boolean copy(int[] x, int a){
       //this method returns true if an integer exists more than once in an array
       int numCount=0;
       boolean more=false;
       for(int thisnum: x){
           if(thisnum==a){
               numCount++;
           }
       }
       if(numCount>1){
           more=true;
       }
       return more;
   }

   public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int _a_size = Integer.parseInt(in.nextLine());
        int[] _a = new int[_a_size];//number of elements in the line
        int _a_item;
        String next = in.nextLine();
        String[] next_split = next.split(" ");

        for(int _a_i = 0; _a_i < _a_size; _a_i++) {
            _a_item = Integer.parseInt(next_split[_a_i]);
            _a[_a_i] = _a_item;
        }
        lonelyinteger(_a);
    }
}