参考答案：

#include<iostream>
using namespace std;
typedef long long LL;
const int N = 1e6 + 10;
LL phi[N], primes[N], st[N];
LL n;
LL cnt, res;
void f(LL n)
{
    phi[1] = 1;
    for (int i = 2; i <= n; ++i)
    {
        if (!st[i])
        {
            primes[++cnt] = i;
            phi[i] = i - 1;
        }
        for (int j = 1; primes[j] <= n / i; ++j)
        {
            st[primes[j] * i] = 1;
            if (i % primes[j] == 0)
            {
                phi[primes[j] * i] = primes[j] * phi[i];
                break;
            }
            else phi[primes[j] * i] = phi[i] * (primes[j] - 1);
        }
    }
}
int main()
{
    cin >> n;
    f(n);
    for (int i = 1; i <= n; ++i) res += phi[i];
    cout << res << endl;
    return 0;
}