Chessboard

import turtle

side_size = int(input('Enter side size: '))
fields_counter = 0
field_size = 40
sides_counter = 0
counter = 0
row_number = 0
is_black = False
turtle.color("black")

while fields_counter < side_size * side_size:
    if side_size % 2 == 0:
        if fields_counter % 2 == 0:
            if row_number % 2 != 0:
                is_black = True
            else:
                is_black = False
        else:
            if row_number % 2 != 0:
                is_black = False
            else:
                is_black = True
    else:
        if is_black:
            is_black = False
        else:
            is_black = True

    if sides_counter < 4:

        if is_black and sides_counter == 0:
            turtle.begin_fill()

        turtle.forward(field_size)
        turtle.left(90)
        sides_counter += 1

        if is_black and sides_counter == 3:
            turtle.end_fill()

    else:
        sides_counter = 0
        fields_counter += 1
        turtle.forward(field_size)

        if fields_counter % side_size == 0:
            row_number += 1
            pos = turtle.pos()
            turtle.penup()
            turtle.goto(pos[0]-(side_size * field_size), pos[1] - field_size)
            turtle.pendown()

turtle.done()