This problem description is lifted from http://en.wikipedia.org/wiki/Levenshtein_distance.

The Levenshtein distance between two strings is defined as the minimum number of edits needed to transform one string into the other, with the allowable edit operations being insertion, deletion, or substitution of a single character.

For example, the Levenshtein distance between "kitten" and "sitting" is 3, since the following three edits change one into the other, and there is no way to do it with fewer than three edits:

 kitten => sitten  (substitution of 's' for 'k')
 sitten => sittin  (substitution of 'e' for 'i')
 sittin => sitting (insert 'g' at the end).

So when

 s1 = 'kitten'

and

 s2 = 'sitting'

then the distance d is equal to 3.