#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 100010;
int q[N];
int main()
{
int n;
scanf("%d", &n);;
for (int i = 0; i < n; i ++ )
scanf("%d", &q[i]);
int res = 0;
sort(q,q+n);
for(int i = n;i>0;i--){
if(q[n-i]>i){
cout<<i;
return 0;
}
}
cout<<"0";
return 0;
}