Lucky Array

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. Chef likes sequences of n positive integers, each of which does not exceed m. The total number of such sequences is equal to m^n. Chef has an array C of n-1 integers. Let F(x) be e