[模板]洛谷T2158 仪仗队 欧拉函数

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

#include<string>
#include<stack>
#include<queue>
#include<vector>
#include<algorithm>
#include<map>
#include<set>

#define inf 2147483647
#define ri register int
#define ll long long

using namespace std;

inline void read(int &x){
    x=0;
    char t=getchar();
    bool f=0;
    
    while(t<'0' || t>'9'){
        if(t=='-')f=1;
        t=getchar();
    }
    
    while(t>='0' && t<='9'){
        x=(x<<3)+(x<<1)+t-'0';
        t=getchar();
    }
    
    if(f)x=-x;
}

inline void Euler();

int a[40005];  //最小质因数 
int pri[10005],p=0;  //质数表 
int f[40005];  //phi

int n,ans=0;

int main(){
    read(n);
    
    n--;
    
    Euler();
    
    for(ri i=2;i<=n;i++)ans+=f[i];
    
    ans=(ans<<1)+3;
    
    printf("%d
",ans);
    
    return 0;
}

inline void Euler(){
    for(ri i=2;i<=n;i++){
        if(!a[i]){
            p++;
            pri[p]=i;
            a[i]=i;
        }
        
        for(ri j=1;j<=p && i*pri[j]<=n;j++){
            a[i*pri[j]]=pri[j];
            if(i%pri[j]==0)break;
        }
    }
    
    f[1]=1;
    
    for(ri i=2;i<=n;i++){
        if(i%(a[i]*a[i])==0)f[i]=f[i/a[i]]*a[i];
        else f[i]=f[i/a[i]]*(a[i]-1);
    }
}