BZOJ2818 Gcd

Description

Input

Output

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
#define rep(i,l,r) for(int i=l; i<=r; i++)
#define clr(x,y) memset(x,y,sizeof(x))
using namespace std;
typedef long long ll;
const int maxn = 10000010;
int n,cnt=0,pri[maxn],phi[maxn];
ll sum[maxn];
bool flag[maxn];
void get_phi(){
clr(flag,0); phi[1] = 1;
rep(i,2,n){
if (!flag[i]) pri[++cnt] = i, phi[i] = i - 1;
for(int j = 1; j <= cnt && i*pri[j] <= n; j++){
flag[i*pri[j]] = 1;
if (i % pri[j]) phi[i*pri[j]] = phi[i] * (pri[j] - 1);
else{
phi[i*pri[j]] = phi[i] * pri[j];
break;
}
}
}
sum[1] = phi[1]; rep(i,2,n) sum[i] = sum[i-1] + phi[i];
}
int main(){
scanf("%d",&n);
get_phi();
ll ans = 0;
rep(i,1,cnt) ans += (sum[n/pri[i]] << 1) - 1;
printf("%lld\n",ans);
return 0;
}