#include<bits/stdc++.h> 
using namespace std;
int n,a[200001];

int main(){
	scanf("%d",&n);
	for (int i=1;i<=n;i++)  scanf("%d",&a[i]);
	sort(a+1,a+n+1);
	int m=unique(a+1,a+n+1)-a-1;
	if (m<=1)  printf("-1");
	else  printf("%d\n",a[m-2]);
	return 0;
}

-1那个点过不了