import math
n, s, ID = map(int, input().split())
a = list(map(int, input().split()))
p = a[ID] + 100 * s
a.pop(ID)
a.sort()
ret = 1.0
for i in reversed(range(len(a))):
    # a[i]+50s+|50s/(0.8+0.2r)|<p
    l = 1
    r = n - 1
    if a[i] + 50 * s + math.floor(50 * s / (0.80 + 0.20 * (n - 1))) > p:
        l = r = n
    if a[i] + 100 * s <= p:
        r = 1
    for t in range(20):
        mid = int((l + r) / 2.0)
        if a[i] + 50 * s + math.floor(50 * s / (0.80 + 0.20 * mid)) <= p:
            r = mid
        else:
            l = mid
    # r位以下ならOK->(i+2-r)/(i+1)
    ret *= max(i + 2 - r, 0) / (i + 1)
print(ret)