#include <cstdio>
#include <cstring>

#include <vector>
#include <string>
#include <stack>
#include <queue>
#include <algorithm>

int main() {

  // step 1
  int N;
  scanf("%d", &N);

  // step 2
  double dp[32]; dp[1] = 1; dp[2] = 2;
  for(int i = 3; i <= N; ++i) {
    double avg1 = 0.0;
    for(int j = 1; j < i; ++j) {
      for(int k = j + 1; k < i; ++k) {
        avg1 += j * k;
      }
    }
    avg1 /= ((i - 1) * (i - 2)) / 2;    
    dp[i] = dp[i - 1] + (avg1 * (i - 2) + 2) / i;
  }

  printf("%.20lf\n", dp[N]);
  
  return 0;
}