OI-codes/Luogu/T243086/T243086.cpp

#include <iostream>

using std::cin;
using std::cout;
const char endl = '\n';

int n, p, primes[10000005], phi[10000005];
bool vis[10000005];
long long ans;

void get_eulers(int n) {
    phi[1] = 1;
    for (int i = 2; i <= n; i++) {
        if (!vis[i]) {
            primes[p++] = i;
            phi[i] = i - 1;
        }
        for (int j = 0; primes[j] * i <= n; j++) {
            vis[primes[j] * i] = true;
            if (i % primes[j] == 0) {
                phi[primes[j] * i] = phi[i] * primes[j];
                break;
            }
            phi[primes[j] * i] = phi[i] * (primes[j] - 1);
        }
    }
}

int main() {
    std::ios::sync_with_stdio(false);

    cin >> n;

    get_eulers(n);

    for (int i = 2; i <= n; i++) {
        ans += phi[i];
    }

    cout << ans << endl;

    return 0;
}