import math
n, s, wid = map(int, input().split())
a = list(map(int, input().split()))
c = [0]*n
tmp = [0]*n
for i in range(n):
	k = 100*s + a[wid] - a[i] + 1
	if k <= 50*s:
		 print(0.0)
		 exit()
	else:
		c[i] = math.floor(1 + ((4*k - 450*s)/(50*s - k)))
		if c[i] < 1:
			c[i] = 1
		tmp[c[i]-1] += 1
for i in range(n-1):
	tmp[i+1] = tmp[i+1] + tmp[i]
for i in range(n):
	a[tmp[c[i]-1]-1] = c[i]
	tmp[c[i]-1] -= 1
ans = 1
for i in range(n-1):
	if n-i-a[n-1-i] > 0:
		ans *= (n-i-a[n-1-i])/(n-1-i)
	else:
		print(0.0)
		exit()
print(ans)