N, S, writer_id = map(int, input().split())
a = list(map(int, input().split()))
W = a[writer_id]+100*S
a = [a[i] for i in range(N) if i!=writer_id]
a.sort(reverse=True)
j = N
ans = 1

for i in range(N-1):
    while a[i]+50*S+250*S//(4+j-1)<=W and j>1:
        j -= 1

    ans *= max(0, N-j-i)
    ans /= N-1-i

print(ans)