def pay(e, h)
  h.default = 0
  h.each {|c,n| e -= c*n }
  raise "Money!" if (e *= -1) < 0
  [500,100,50,10,5,1].each do |c|
    h[c] -= e/c
    e %= c
  end
  h.reject {|c,n| n <= 0}
end