#!/usr/bin/env python3
n, s, writer_id = map(int, input().split())
a = list(map(int, input().split()))
score = lambda r: 50 * s + 250 * s // (4 + r)
self = a[writer_id] + score(1)
del a[writer_id]
a.sort()
p = 1.0
i = 0
for r in range(1, n):
    while i < len(a) and a[i] + score(r) <= self:
        i += 1
    p *= max(0, i-(r-1)) / (n-1 - (r-1))
print('%.12f' % p)