import bisect

def yukicoder(s,r):
	base=50*s
	bonus=500*s/(8+2*r)
	return base+bonus

n,s,writer_id = list(map(int, input().split()))	
a = list(map(int, input().split()))
writer_score=a[writer_id]+100*s;

b=[]
for i in range(n):
	if i!=writer_id:
		b.append(a[i])

b.sort()

ans=1.0;
total=n-1;

for solved in range(total):
	add_score=yukicoder(s,solved+1);
	valid=bisect.bisect_right(b,writer_score-add_score)
	ans*=1.0*(valid-solved)/(total-solved);	

print(ans)