Lucky Count

Chef loves lucky numbers. Everybody knows that lucky numbers are positive integers whose decimal representation contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not. Let Fd(x) equals to the number of digits d in decimal representation of the positive integer x. Chef interests only in functions F4(x) and F7(x). For the given positive integer N he