Lucky String

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 also use term "lucky sum". Lucky sum is an operation between two integers. Let the first integer is A, A[i] equals i-th digit of A (0-base numeration, from right to left) and t