import sys
input = lambda : sys.stdin.readline().rstrip()

sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))


n,s,wid = list(map(int, input().split()))
a = list(map(int, input().split()))
w = a[wid] + 100*s
a.pop(wid)
a.sort(reverse=1)
vals = [int(50*s + (50*s / (0.8 + 0.2*i))) for i in range(1, n)[::-1]]
from bisect import bisect_right as br
ans = 1
j = 0
l = list(range(1,n))
for i in range(n-1):
    v = br(vals, w - a[i])
    if v<=i:
        ans = 0
        break
    ans *= (v-i)
    while j<len(l) and ans>l[j]:
        ans /= l[j]
        j += 1
for k in range(j, len(l)):
    ans /= l[k]
print(ans)