题解

#include <bits/stdc++.h>
using namespace std;
bool prime[2000010];
void check(){
    prime[0]=1;
    prime[1]=1;
    for(long long i=2;i<=2000010;i++){
        if(prime[i]==0){
            for(long long j=i*i;j<=2000010;j+=i){
                prime[j]=1;
            }
        }
    }
}
int main(){
    long long n,ans=0;
    cin>>n;
    check();
    for(int i=1;i<=n;i++)if(prime[i]==0)ans++;
    cout<<ans;
    return 0;
}