/*
TASK:edi
LANG:C++
*/
#include<iostream>
using namespace std;
int b[10001];
int main()
{
long n,a,i,j,m=1;
cin>>n;
for(i=1;i<=n;i++){
                  cin>>a;
                  for(j=m;j<=a;j++){b[j]++;if(b[i]>=i)m=i;}
                  }
for(i=1;i<=10000;i++)if((b[i]>=i)&&(i>m))m=i;
cout<<m<<"\n";             
 return 0;
}
