#include<stdio.h>
int main()
{
    int n,i,x,y;
    while(scanf("%d",&n)!=EOF&&n)
    {
           x=0;
           while(n--)
           {
                scanf("%d",&y);
                x^=y;
           }
           printf("%d\n",x);
    }
    return 0;
}

#include <stdio.h>  
#include <algorithm>  
#include <map>  
using namespace std;  
  
int main()  
{  
    int n,i,a;  
    map<int,int>my;  
    map<int,int>::iterator it;  
    while(scanf("%d",&n),n)  
    {  
        for(i=0;i<n;i++)   
        {  
            scanf("%d",&a);  
            my[a]++;  
        }  
        for(it=my.begin();it!=my.end();it++)  
        {  
            if(it->second==1)  
            {  
                printf("%d\n",it->first);  
                break;  
            }  
        }  
        my.clear();  
    }  
    return 0;  
}