DEBUG = true
MOD = 10**9+7

def main
  _N, _M, _C = ints
  a = ints.sort.reverse
  b = ints.sort

  cnt = 0
  j = 0
  a.each do |x|
    j += 1 while j < _M and x * b[j] <= _C
    cnt += _M - j
  end

  puts cnt.to_f / (_N * _M)
end

def int; gets.to_i; end
def ints; gets.split.map &:to_i; end
def string; gets.chomp; end
def rep(n, &b); n.times.map(&b); end
def yes; puts "Yes"; end
def no; puts "No"; end
def yesno t; t ? Yes : No; end
def debug(x); STDERR.puts x.inspect if DEBUG; end

def factorial(n, mod); (2..n).inject(1) { |f, x| f * x % mod }; end
def cumsum(xs); ys = [0]; xs.each { |x| ys << x + ys[-1] }; ys; end
def cumdiff(ys); xs = []; xs.inject { |x, y| xs << (d = y - x); d }; end
def mod_inv(x, mod); x.pow(mod-2, mod); end
def mod_div(x, y, mod); x * mod_inv(y, mod) % mod; end

main