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

int main(){
    int n; cin >> n;
    set<int> a;
    for(int i = 0; i < n; i++){
        int k; cin >> k;
        a.insert(k);
    }
    int train = 1, ans = 0;
    for(auto it = a.begin(); it != a.end(); it++){
        if(train <= (*it)){ ans++; }
        train++;
        //cout << *it << ' ';
    }
    cout << ans;
}