#include<iostream>
#include<algorithm>
using namespace std;

const int MAXN = 2e5 + 10;
int a[MAXN],n,cnt;
bool flag;

int main(){
	cin >> n >> a[1];
	for(int i = 2;i <= n;++i){
		cin >> a[i];
		if(a[i] != a[1]) flag = 1;
	}
	if(!flag){
		cout << "-1" << endl;
		return 0;
	}
	sort(a + 1,a + n + 1);
	for(int i = n - 1;i >= 1;--i){
		if(a[i] != a[i + 1]) cnt++;
		if(cnt == 2){
			cout << a[i] << endl;
			return 0;
		}
	}
	return 0;
}