A beginner in programming, I decided to create a program that creates a graph of the root function. There was an error in the tkinter module. How to fix?

Question

Python

Here is the code that should create the graph:

from tkinter import Tk, Canvas, Frame, BOTH
from math import *
from re import *

win = Tk()
win.geometry('500x500')
win.resizable(False, False)
c = input()
x = []
if search(r'\bsqrt\b', c):
    a = search(r'\d*', c)
    for i in range(501):
        x.append((i+3))
        x.append(400 - (sqrt(i)*a[0]))

canvas = Canvas()
canvas.create_line(3, 400, 503, 400, fill = 'red', width = 2)
canvas.create_line(3, 400, 3, 0, fill = 'red', width = 2)
canvas.create_polygon(x, outline = 'red', fill = 'white', width = 2)
canvas.pack(fill=BOTH, expand=1)

win.mainloop()

The following error occurs when entering 2sqrt:

Traceback (most recent call last):
  File "C:\Python32\aert", line 23, in <module>
    canvas.create_polygon(x, outline = 'red', fill = 'white', width = 2)
  File "C:\Python32\lib\tkinter\__init__.py", line 2191, in create_polygon
    return self._create('polygon', args, kw)
  File "C:\Python32\lib\tkinter\__init__.py", line 2166, in _create
    cnf = args[-1]
IndexError: tuple index out of range

How to fix a mistake, if not difficult - with an explanation