function! s:output()
  let this = {'lines': []}

  function! this.append(line) abort
    call add(self.lines, a:line)
  endfunction

  function! this.build() abort
    return join(self.lines, "\n")
  endfunction

  return this
endfunction

function! s:main(input) abort
  let o = s:output()

  let s = a:input[0]
  let r = []
  let offset = char2nr('A')
  for i in range(strlen(s))
    let ord = char2nr(s[i]) - offset
    let moved = (26 + ord - i % 26 - 1) % 26
    let chr = nr2char(moved + offset)
    call add(r, chr)
  endfor

  call o.append(join(r, ''))
  return o.build()
endfunction

let s:input = getline(1, '$')
enew
put =s:main(s:input)
2,$print