   1 LET X=30
   2 LET Y=40
   3 LET K=INT (RND*3)
   4 LET A=30*K
   5 LET B=40*(K=1)
   6 LET X=INT ((X+A)/2)
   7 LET Y=INT ((Y+B)/2)
   8 PLOT X,Y
   9 GOTO 3
  10 REM MARCO VARESIO 2020
