main = solve . head . map read . words <$> getLine >>= putStrLn

cs = ['a'..'z']

at 1 (c:_) = c
at n (_:l) = at (n-1) l
at _ []    = '_'

solve n = reverse $ solve' n

solve' :: Integer -> String
solve' n
    | n <= 26    = [at n cs]
    | otherwise = at n' cs : solve' n''
    where
        n'  = n `mod` 26
        n'' = n `div` 26