import sys
from bisect import bisect_left
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))


N,M,C = MI()
A,B = LI(),LI()
B.sort()

ans = 0
for a in A:
    x = (C//a)+1
    index = bisect_left(B,x)
    ans += M-index
print(ans/(N*M))