/* 1776. Anniversary Firework - http://acm.timus.ru/problem.aspx?num=1776
 *
 * Strategy:
 * Dynamic programming. Keep track of the probability of finishing a line of rockets after a certain
 * number of salvos (at an exact number of salvos or cumulatively up until that amount of salvos),
 * and build a solution on this, bottom up.
 *
 * Performance:
 * O(n^3), running the test cases in 0.187s using 2704KB memory.
 */

#include <stdio.h>

double A[401][401]; // Probability of finishing n rockets at exactly k salvos
double C[401][401]; // Probability of finishing n rockets after at most k salvos

inline void calcKs(int N, int n, int i)
{
    double c = 0.0;
    for(int k = 1; k < 401; k++)
    {
        double p = 1/double(N) * (A[n][k-1]*C[i][k-1] + C[n][k-1]*A[i][k-1] - A[n][k-1]*A[i][k-1]);
        A[N][k] += p;
        c += p;
        C[N][k] += c;
    }
}

int main()
{
    int N;
    for(int i = 0; i < 401; i++)
        C[1][i] = C[0][i] = 1.0;
    A[1][0] = 1.0;
    A[0][0] = 1.0;

    scanf("%d", &N);
    for(int n = 2; n <= N-2; n++)
        for(int i = 0; i < n; i++)
            calcKs(n, n-1-i, i);
    double ans = 0.0;
    for(int i = 0; i < 401; i++)
        ans += A[N-2][i]*10.0*i;
    printf("%.10lf", ans+10.0);
}