luogu_2158 [SDOI2008]仪仗队

#include <cstdio>
#include <iostream>
using namespace std;
const int N=40010;
int n,p[N],nop[N],cnt,phi[N],ans;

int main(){
    scanf("%d",&n);
    if(n==0){puts("0"); return 0;}
    nop[1]=1;
    for(int i=2;i<=n;i++){
        if(!nop[i]){p[++cnt]=i; phi[i]=i-1;}
        for(int j=1;j<=cnt && i*p[j]<=n;j++){
            nop[i*p[j]]=1;
            if(i%p[j]==0){phi[i*p[j]]=phi[i]*p[j]; break;}
            else phi[i*p[j]]=phi[i]*(p[j]-1);
        }
    }
    for(int i=2;i<=n-1;i++)ans+=phi[i];
    printf("%d\n",ans*2+3);
    return 0;
}