There is a string of length N made only of letters $"$a$".$ Whenever there are two identical adjacent letters $($e$.$g$."$aa$"),$ they can be transformed into a single letter that is the next letter of the alphabet. For example, $"$aa$"$ can be transformed into $"$b$"$ and $"$ee$"$ into $"$f$".$ However, $"$zz$"$ cannot be further transformed.

What is the alphabetically largest string that can be obtained from the initial string?

Examples:

$1.$ Given N $=11,$ the function should return "dba". The initial string

"aaaaaaaaaaa" can be transformed in the following manner:

'aaaaaaaaaaa" $->$ "bbbbba"$>$ ccba" $->$ "dba"

$2.$ Given N $=1,$ the function should return $"$a$".$ The initial string $"$a$\text{'}$

cannot be transformed in any way.

$3.$ Given N $=67108876,$ the function should return $"$zzdc$"$

Write an efficient algorithm for the following assumptions:

$$ N is an integer within the range $[1\mathrm{.}1,000,000,000).$