def binary_search(ai):
    left, right = 1, N-1
    
    while left<=right:
        mid = (left+right)//2
        
        if ai+50*S+50*S//(0.8+0.2*mid)<=W:
            right = mid-1
        else:
            left = mid+1
    
    return left

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)
ans = 1

for i in range(N-1):
    j = binary_search(a[i])
    ans *= max(0, N-j-i)
    ans /= N-1-i
    
print(ans)