I write:
Var f,ff:Text;
    A,B,N:byte; R:Longint;
Begin
  Assign (f,'input.txt');
  Reset (f);
  Read (f,N,A,B);
  close(f);
  r:=2*A*B*n;
  Assign (ff,'output.txt');
  Rewrite (ff);
  Write (ff,r);
  close (ff);
end.
Why wrong answer? Help me, please...

Try to read and write not to files. Use standart stream(read from keyboard, write to monitor). Judge system automaticaly change standart stream to files.