import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;

public class Main{
    public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String line[] = r.readLine().split(" ");
        int n = Integer.valueOf(line[0]);
        String line2[] = r.readLine().split(" ");
        int[] arr = new int[n];
        for (int i = 0; i <n ; i++) {
            arr[i] = Integer.valueOf(line2[i]);
        }
        Arrays.sort(arr);//先对数组进行排序
        int left = 0;
        int right = arr[arr.length-1];
        int res = 0;
        while(left<right){
            int mid = (left+right)/2+1;//找到中间数
            int cnt = 0;
            for (int i = 0; i <arr.length ; i++) {
                if(arr[i]>=mid)//看看有几个论文超过了他
                    cnt++;
            }
            if(cnt>=mid){//如果H指数超过的之前的话 置新的
                res = mid; //7个5 还是置5
                left = mid;
            }
            else{
                right = mid-1;
            }
        }

        System.out.println(res);
    }


}