Write a method that swaps pairs of letters from the input parameter string. As a precondition, you may assume that the input parameter string contains an even number of characters. So "this" is a possible input parameter string, but "not" is not a possible input parameter string because it has an odd number of characters. Here is an example: Passing in "abcd" produces a result of "badc" because the a and b are swapped, then the c and d are swapped.