K = input()

def pred(e0):
    E = [1] * (K+1)
    E[K] = 0
    for x in xrange(K-1, -1, -1):
        for y in xrange(x+1, x+7):
            E[x] += (e0 if y > K else E[y]) / 6.0
    return E[0]


def solve():
    lb, ub = 1, K * 6
    for i in range(40):
        m = (lb + ub) / 2.0
        p = pred(m)
        if p < m:
            ub = m
        else:
            lb = m
    return (lb + ub) / 2

print(solve())