Extracting A File Name From A Path

The formula which I create extracts the last word from a string. However it won't work. Why?

Formula:

=RIGHT(B11, LEN(B11)-FIND(" ", B11, LEN(B11)-LEN(SUBSTITUTE(B11, " ", ""))))

1)The second argument of it works in this way;

It substracts the position of last space from the number of all characters. So it finds the character number which last word contains.

2) It finds last space in this way;

It substracts the number of all characters without spaces from the all characters.