P2158 [SDOI2008]仪仗队

https://www.luogu.com.cn/problem/P2158

#include <bits/stdc++.h>

using namespace std;
#define int long long
const int maxn = 4e4 + 10;
int v[maxn], phi[maxn], prime[maxn];
int n, m, ans;

void euler() {
    m = 0;
    phi[1] = 1;
    for (int i = 2; i < n; i++) {
        if (!v[i]) v[i] = 1, prime[++m] = i, phi[i] = i - 1;
        for (int j = 1; j <= m && i * prime[j] < n; j++) {
            v[i * prime[j]] = 1;
            phi[i * prime[j]] = phi[i] * (i % prime[j] ? prime[j] - 1 : prime[j]);
            if (i % prime[j] == 0) break;
        }
    }
}

signed main() {
    //freopen("in","r",stdin);
    ios::sync_with_stdio(0);
    cin >> n;
    if(n == 1) cout << 0;
    else {
        euler();
        ans = 0;
        for (int j = 2; j < n; j++) ans += phi[j];
        cout << 2 * ans + 3 ;
    }

    return 0;
}