Write a function that takes in two strings and returns the minimum number of operations required to convert the first string into the second string. The available operations are: insert a character, delete a character, and replace a character. Each operation counts as one unit. For example, if the input strings are "kitten" and "sitting", the function should return 3, because it is possible to convert "kitten" to "sitting" by replacing "k" with "s", replacing "e" with "i", and inserting a "g" at the end.