C++ :

#include<bits/stdc++.h>
using namespace std;
int n,x,a[1000001],b[1001],ans,t;
int main()
{
//	freopen("1018.in","r",stdin);
//	freopen("1018.out","w",stdout);	
	cin>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>x;
		a[i]=x;
		b[x]++;
	}
	for(int i=1;i<=n;i++)
	{
		if(b[a[i]]==1) {t=1;ans=a[i];break;}
	}
	if(t==1) cout<<ans;
	else cout<<0;
	return 0;
}