以下是代码

#include<bits/stdc++.h>
using namespace std;
const unsigned int N=(int)1e8+10;
unsigned int a[N],b[N];
int main(){
	unsigned n,t=0;
	scanf("%u",&n);
	for(unsigned int i=2;i<=n;i++){
		if(!a[i]) b[++t]=i;
		for(unsigned int j=1;b[j]*i<=n;j++){
			a[b[j]*i]=1;
			if(i%b[j]==0){
				break;
			}
		}
	}
	printf("%u",t);
	return 0;
}