//二分

#include <iostream>

using namespace std;

const int N = 100010;

int n;
int w[N];

bool check(int x){
    int a = 0;
    for(int i=0;i<n;i++){
        if(w[i] >= x)
            a++;
    }

    return a >= x;
}

int main(){

    cin >> n;
    for(int i=0;i<n;i++){
        cin >> w[i];
    }

    int l=0,r=n;
    while(l<r){
        int mid = (l + r + 1) >> 1;
        if(check(mid)){
            l = mid;
        }else{
            r = mid-1;
        }
    }

    cout <<  r << endl;

    return 0;
}