題目:
題解:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll N=1e7+10;
int pri[N],s[N],st[N],con,primes[N];
void oula(ll n)
{
pri[1]=1;
for(ll i=2;i<=n;i++)
{
if(!st[i])
{
pri[i]=i-1;
primes[++con]=i;
}
for(ll j=1;primes[j]<=n/i;j++)
{
ll t=primes[j]*i;
st[t]=1;
if(i%primes[j]==0)
{
pri[t]=primes[j]*pri[i];
break;
}
pri[t]=pri[i]*(primes[j]-1);
}
}
for(int i=2;i<=n;i++) s[i]=s[i-1]+pri[i];
}
int main()
{
ll n;
cin>>n;
oula(n);
ll ans=0;
for(int i=1;i<=con;i++)
{
ll p=primes[i];
ans+=2*s[n/p]+1;
}
cout<<ans<<endl;
return 0;
}