BZOJ 2190 仪仗队

#include <iostream>
#include <cstdio> 
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <cstring>

#define ll long long 
#define ull unsigned long long
 
using namespace std;
 
bool f[40001];
ull p[40001];
ull ans[40001];
  
int main()
{ 
    int n,tot=0;
    cin >>n;
    if (n<=1)
    { 
        cout <<0 <<endl;
        return 0;
    }
 
    memset(f,false,sizeof(f));
    f[1]=true;
    for (int i=2;i<=n;i++)
    {
        if (!f[i])
        {
            p[++tot]=i;
            ans[i]=i-1;
        }
        for (int j=1;(j<=tot)&&(i*p[j]<=n);j++)
        {
            f[i*p[j]]=true;
            if (i%p[j]==0)
            {
                    ans[i*p[j]]=ans[i]*p[j];
                    break;
            } 
            else
            {
                ans[i*p[j]]=ans[i]*(p[j]-1);
            }
        }
 
    }
 
    long long pr=2;
    for (int i=2;i<=n-1;i++) pr+=ans[i];
 
    cout <<pr*2-1 <<endl;
 
    return 0;
 }