#include<iostream>
using namespace std;
int main()
{
int n,x,y;
while(cin>>n,n)
{
cin>>x;
while(--n)
{
cin>>y;
x=x^y;
}
cout<<x<<"
";
}
return 0;
}
#include<iostream>
using namespace std;
int main()
{
int n,x,y;
while(cin>>n,n)
{
cin>>x;
while(--n)
{
cin>>y;
x=x^y;
}
cout<<x<<"
";
}
return 0;
}