ocaml, 148 bytes

Result

AC

Duration

796ms

Code [DL]

let rec f x=let c=input_char stdin in
let x=if c='T' then 51 else if c='K' then x-50 else (x+1)mod 51 in
print_char(if x<1 then 'x' else c);f x;;f 0

stdin

          T                                       
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
    K                                             

stdout

          T                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
          x                                       
    xxxxxxx                                       

stderr

Exception: End_of_file.

strace