Find the lexicographical smallest permutation of a string that contains of 'a' and 'b' only such that no. of subsequence of "ab" is x and of "ba" is y

Given a string of ‘a’ and ‘b’ only

for example
string is ‘baaab’ and x=3 and y=3

Find the lexicographical smallest permutation of string that contains

count of sub-sequence ‘ab’ = x times and count of sub-sequence ‘ba’ = y times.

For above example , there is a possible permutation ababa

We can print -1 , in case if its not possible

for example when string is ‘abbb’ and x = 3 and y = 3.

For this example no permutation is possible

What logic will you use ?