使用欧拉函数
#include <bits/stdc++.h>
using namespace std;
bool isprime[10000005];
int cnt = 0, pr[10000005];
int phi[10000005];
long long sum[10000005];
int main() {
int n;
scanf("%d", &n);
memset(isprime, true, sizeof(isprime));
isprime[1] = false;
phi[1] = 1;
for (int i = 2; i <= n; i++) {
if (isprime[i]) {
pr[++cnt] = i;
phi[i] = i - 1;
}
for (int j = 1; j <= cnt && i * pr[j] <= n; j++) {
isprime[pr[j] * i] = false;
if (__gcd(pr[j], i) == 1)
phi[pr[j] * i] = phi[pr[j]] * phi[i];
else
phi[pr[j] * i] = pr[j] * phi[i];
if (i % pr[j] == 0)
break;
}
}
for (int i = 1; i <= n; ++i)
{
sum[i] = sum[i - 1] + phi[i];
}
long long ans = 0;
for (int i = 1; i <= cnt && pr[i] <= n; ++i)
{
ans += (sum[n / pr[i]] << 1) - 1;
}
printf("%lld\n", ans);
return 0;
}