#!/usr/bin/env python3

from fractions import Fraction

def pick(state, depth, numofc, cache={}):
    if sum(state) == depth:
        return len(state) - state.count(0)
    if state in cache:
        return cache[state]
    r = 0
    for c, num in enumerate(state):
        nws = sorted(state[:c] + (num + 1,) + state[c+1:])
        nws = tuple(nws)
        r += Fraction(numofc-num, numofc*len(state) - sum(state)) * \
             pick(nws, depth, numofc, cache)
    cache[state] = r
    return r

r = pick(7*(0,), 20, 10)
print(float(round(r, 9)))