elixir, 154 bytes

Result

AC

Duration

2180ms

Code [DL]

defmodule M do
def f(x)do
c=IO.read(1)
x=case c do
:eof->exit(0)
"T"->51
"K"->x-50
_->rem(x+1,51)
end
IO.write(if x<1,do: "@",else: c)
f(x)
end
end
M.f(0)

stdin

                   T                              
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
        K                                         

stdout

                   T                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
                   @                              
        @@@@@@@@@@@@                              

stderr

** (exit) 0
    /volume/CODE:5: M.f/1
    (elixir) lib/code.ex:767: Code.require_file/2

strace