#!/usr/bin/python2
# -*- coding: utf-8 -*-
# †
from bisect import bisect_right

n, s, writer_id = map(int, raw_input().split())
a = map(int, raw_input().split())
score = []
for i in xrange(1, n):
    score.append(s * 50 + (s * 250) / (i + 4))
writer_score = a[writer_id] + score[0]
score.reverse()

posArr = []
for i in xrange(n):
    if i == writer_id:
        continue
    pos = bisect_right(score, writer_score - a[i])
    posArr.append(pos)

posArr.sort()
prob = 1.
for i in xrange(n-1):
    prob *= float(posArr[i] - i) / (n - i - 1)
print '{:.15f}'.format(abs(prob))