Recamán sequence generator in Python 3

#Importing time so you can see how long generation took
import time

#Screen size finder shamelessly stolen from StackOverflow
import ctypes
user32=ctypes.windll.user32
width=user32.GetSystemMetrics(0)
height=user32.GetSystemMetrics(1)
middle=height/2

#Preparing variables for sequence generator
#limit is how far the sequence is generated for
print("Recaman sequence generator by u/-Kasai-")
limit=int(input("How many numbers do you want generated? "))
sequence=[0]
location=0

#Generator loop
print("Generating sequence...")
start_time=time.time()
for i in range(1,limit):
    if location-i>0 and location-i not in sequence:
        location-=i
        sequence.append(location)
    else:
        location+=i
        sequence.append(location)
#print(sequence)
#^ You can delete the hashtag above and see the sequence in the shell

#Zoom dictates the pixel to number ratio
#Zoom is auto-fit to sequence limit. If you wanted, you could continue the sequence for a true image, but things get a bit messy
#Originally zoom was based off the size of the sequence. It's now set to just fit the sequence from end to end
zoom=width/max(sequence)

#Drawing the tkinter canvas
print("Drawing...")
from tkinter import *
Tk=Tk()
Screen=Canvas(Tk,width=width,height=height)
#Adding white background
Screen.create_rectangle(0,0,width,height,fill='#ffffff')
Screen.pack()
#This function draws the semicircles that make up the pattern
def semicircle(start,end,direction):
    if direction=='up':
        starting=0
    elif direction=='down':
        starting=180
    #The middle variable here is half the screen height, as defined earlier. Changing the middle variable will shift the sequence up or down vertically
    Screen.create_arc(start,middle-abs((start-end)/2),end,middle+abs((start-end)/2),style=ARC,start=starting,extent=180)

#This variable dictates whether the first step will go up or down. Numberphile goes down. Change it to 'up' if you want!
direct='down'
#This loop draws the pattern
for i in range(len(sequence)-1):
    semicircle(sequence[i]*zoom,sequence[i+1]*zoom,direct)
    if direct=='up':
        direct='down'
    else:
        direct='up'

#Finding the ending time
end_time=time.time()

print("Done!")
print("Generated and drawn in", round(end_time-start_time,4), "seconds.")
print("Drawing should be in another window.")
print("\nProgram created by u/-Kasai-")