#include <bits/stdc++.h>
using namespace std;
int n;
bool jack[10000005];
void jackfather(int n)
{
	memset(jack,true,sizeof(jack));
	jack[0]=jack[1]=false;
	int cnt=(int )sqrt(n+0.5)+1;
	 for(int i=2;i<cnt+1;i++)
	 {
			if(!jack[i])
			{
				continue;	
			}
			for(int j=i+i;j<n+1;j+=i)
			{
				jack[j]=false;
			}
	} 
	return;
}
int main()
{
	int jac[10000005];
	cin>>n;
	int s=0;
	for(int i=1;i<=n;i++)
	{
		jac[i]=i;

		} 
			jackfather(n);
	for(int i=1;i<=n;i++)
	{
		if(jack[i]==1) s++;
		//cout<<jac[i]<<" ";
		}
		cout<<s;
	return 0;
}

but why！