#include<stdio.h>
#include<string.h>
#include<stdlib.h>
int cmp(const void *a,const void *b)
{
return *(int*)a-*(int*)b;
}
int num[1000100];
int main()
{
int n;
int i;
while(scanf("%d",&n)!=EOF)
{
if(n==0)
break;
for(i=0;i<=1000000;i++)
num[i]=-1;
for(i=0;i<=n-1;i++)
scanf("%d",&num[i]);
qsort(num,n,sizeof(num[0]),cmp);
for(i=0;i<=n-1; )
{
if(num[i]==num[i+1])
i=i+2;
else
{
printf("%d\n",num[i]);
break;
}
}
}
return 0;
}