筛质数

暴力遍历判断质数即可：

import java.util.*;
public class Main {
    public static void main(String[] args) throws Exception{
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int r = 0;
        for(int i = 2; i < N; ++i){
            if(f(i)) r++;
        }
        System.out.println(r);
    }
    static boolean f(int n){    //遍历判断质数
        if(n <= 1) return false;
        if(n == 2) return true;
        for(int i = 2; i < n; ++i){
            if(n%i == 0) return false;
        }
        return true;
    }
}