import bisect
N,M,C = map(int,input().split()); Error = pow(10,-9); INF = float("inf")
A = list(map(int,input().split()))
B = list(map(int,input().split()))
B = [-INF] + B + [INF]
B.sort()
ALL = N*M
cnt = 0
for i in range(N):
  ok = M+1
  ng = 0
  while abs(ok-ng) > 1:
    mid = (ok+ng)//2
    if B[mid]*A[i] > C:
      ok = mid
    else:
      ng = mid
  cnt += M+1 - ok
#print(cnt)
ans = cnt/ALL
print(ans)