【BZOJ 2190】【SDOI 2008】仪仗队 欧拉筛

#include<cstdio>
using namespace std;
const int N=40003;
int num=0,prime[N],phi[N];
bool notp[N];
inline void shai(int n){
    phi[1]=1;
    for(int i=2;i<=n;++i){
        if (!notp[i]){
            prime[++num]=i;
            phi[i]=i-1;
        }
        for(int j=1;j<=num&&i*prime[j]<=n;++j){
            notp[i*prime[j]]=1;
            if (i%prime[j]==0){
                phi[i*prime[j]]=phi[i]*prime[j];
                break;
            }else
                phi[i*prime[j]]=phi[i]*phi[prime[j]];
        }
    }
}
int main(){
	int n;
	scanf("%d\n",&n);
    shai(n-1);
    int ans=0;
    for(int i=1;i<n;++i)
    	ans+=phi[i];
    printf("%d\n",ans*2+1);
    return 0;
}