/*
TASK:edi
LANG:C++
*/
#include<iostream>
using namespace std;
int n,i,j,k,p=1;
int a[100000];
int main()
{
    cin>>n;
    for(i=0;i<n;i++)
    cin>>a[i];
    sort(a,a+n);
    for(i=(n-1);i>=0;i--)
    {
                k=n-(i+1);
                k+=1;
                if(k>=a[i]){cout<<a[i]<<"\n";p=0;break;}    
                k=0;
    }
    if(p)cout<<"0\n";
}
