import sys
readline=sys.stdin.readline

N,S,i=map(int,readline().split())
A=list(map(int,readline().split()))
points=[]
for R in range(1,N):
    points.append(50*S+500*S//(8+2*R))
P=A[i]+100*S
A=A[:i]+A[i+1:]
A.sort(reverse=True)
ans=1.0
points.sort()
i=0
for j,a in enumerate(A):
    while i!=N-1 and points[i]+a<=P:
        i+=1
    ans*=(i-j)/(N-j-1)
print(ans)