Little Elephant and Filling

A Little Elephant from the Zoo of Lviv likes lucky strings, i.e., the strings that consist only of the lucky digits 4 and 7. The Little Elephant has the string S such that each character in S is either the lucky digit (4 or 7) or the question mark ?. He can replace each question mark with one of the lucky digits in order to obtain the lucky string. He wants to know the number of different ways he