Little Elephant and Swapping

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. He is now studying some special transformation defined on the set of the lucky strings. The lucky string T is called a swap permutation of the lucky string S with |S| = N if it can be derived from S by the following process. - Choose some integers L and R such that 1 ≤ L ≤