i.cpp

#include <bits/stdc++.h>
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define ll long long

#define IOS ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);

using namespace std;
const int INF = 1e9 + 1;
const int MOD = 1e9 + 7;
const int MAXN = 1e6 + 10;
int cnt[MAXN], a[MAXN], s[MAXN], kk[MAXN];
int min_div[MAXN + 10];
ll dp[MAXN + 10], phi[MAXN + 10];
void solve(){

    ll n;
    cin >> n;

    cout << 1LL * n * (n - 1) / 2 - dp[n] << "\n";

}

int main() {
    IOS;

    for (int i = 2; i <= MAXN; i++){
        if (min_div[i] == 0){
            min_div[i] = i;
            for (ll j = 1LL * i * i; j <= MAXN; j += i){
                if (min_div[j] == 0){
                    min_div[j] = i;
                }
            }
        }
    }
    phi[1] = 0;
    dp[1] = 0;
    for (int i = 2; i <= MAXN; i++){
        if (min_div[i] == i){
            phi[i] = i - 1;
        }
        else{
            int x = i;
            int y = 1;
            while(x % min_div[i] == 0){
                x /= min_div[i];
                y *= min_div[i];
            }
            if (x == 1){
                phi[i] = i - i / min_div[i];
            }
            else{
                phi[i] = phi[x] * phi[y];
            }
        }
        dp[i] = dp[i - 1] + phi[i];
    }

    int t = 1;
   // cin >> t;

    while(t--){
        solve();
    }
    return 0;
}


//phi[120] = phi[2 ^ 3] * phi[3] * phi[5]