#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int N=2e5+5;
bool cmp(ll x,ll y) {
	return x>y;
}
ll n,k;
ll a[N],t[N];
int main() {
	cin>>n;
	for (int i=1; i<=n; i++) {
		cin>>a[i];
	}
	sort(a+1,a+n+1,cmp);
	ll x=unique(a+1,a+n+1)-a-1;
	if(x<2){
		cout<<"-1";
		exit(0);
	}
	cout<<a[3];
	return 0;
}