#!/usr/bin/env python3.8
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines


N, S, writer, *A = map(int, read().split())

me = A[writer] + 100 * S
INF = N + 10
R = [0] * N
for i, x in enumerate(A):
    if i == writer:
        continue
    RHS = me + 1 - x - 50 * S
    if RHS <= 0:
        print(0)
        exit()
    R[i] = 250 * S // RHS - 3

R = [x for i, x in enumerate(R) if i != writer]
R.sort()
R.append(N+100)
cand = 0
i = 0
p = 1
for n in range(1, N):
    while R[i] <= n:
        i += 1
        cand += 1
    p *= cand
    p /= (N - n)
    cand -= 1

print(p)