#include <bits/stdc++.h>
using namespace std;
const int MAX = 1000000;
vector<long double> sum(MAX + 1);
const long double GAMMA = 0.577215664901532;
void precalc(){
  sum[0] = 0;
  for (int i = 1; i <= MAX; i++){
    sum[i] = sum[i - 1] + (long double) 1 / i;
  }
}
long double harmonic(long long N){
  if (N <= MAX){
    return sum[N];
  } else {
    return logl(N) + GAMMA + (double) 1 / (N * 2) - 1 / (12 * pow(N, 2));
  }
}
vector<pair<long long, pair<long long, long long>>> quotient_ranges(long long N){
  vector<pair<long long, pair<long long, long long>>> ans;
  for (long long i = 1; i * i <= N; i++){
    ans.push_back(make_pair(N / i, make_pair(i, i)));
  }
  for (long long i = N / ((long long) sqrt(N) + 1); i >= 1; i--){
    ans.push_back(make_pair(i, make_pair(N / (i + 1) + 1, N / i)));
  }
  return ans;
}
int main(){
  precalc();
  cout << fixed << setprecision(20);
  long long N;
  cin >> N;
  vector<pair<long long, pair<long long, long long>>> Q = quotient_ranges(N);
  int cnt = Q.size();
  long double ans = 0;
  for (int i = 0; i < cnt; i++){
    long long q = Q[i].first;
    long long mn = Q[i].second.first;
    long long mx = Q[i].second.second;
    ans += harmonic(q) * (harmonic(mx) - harmonic(mn - 1));
  }
  cout << ans << endl;
}