import sequtils,strutils
var
    K = stdin.readline.parseInt
    P : array[21,array[130,float]]
    ans : float
P[0][0] = 1
for i in 0..<K:
    for j in i..(6 * i):
        for k in 1..6:
            if j + k < K:
                P[i + 1][j + k] += P[i][j] / 6
            else:
                ans += P[i][j] / 6 * (i + 1).float
echo ans