OBJECTIVE:Pen up and down

CODE:

import turtle
rehan=turtle.Turtle ()
rehan.speed (0)
for i in range (4):
rehan.pencolor("green")
rehan.forward (100)
rehan.left (90)
rehan=turtle.Screen ()
rehan=turtle.Turtle ()
rehan.pencolor ("pink")
rehan.penup ()
rehan.goto (-100,50)
rehan.pendown ()
rehan.circle(50)