#include<bits/stdc++.h>
using namespace std;
int n,z,a[1000001];
int main(){
cin>>n;
for(int i=1;i<=n;i++) 
cin>>a[i];
sort(a+1,a+n+1);
z=unique(a+1,a+n+1)-a;
if(z<=2) 
cout<<"-1"<<endl;
	else 
cout<<a[z-3]<<endl;
return 0;
}