counting chars in string w/o len

[johneven]

Here's my homework assignment:

Write code to count the number of characters in original_str using for loop as a counter and assign the answer to a variable num_chars. Do NOT use the len function to solve the problem.

#Here's my attempt:

original_str = "The quick brown rhino jumped over the extremely lazy fox."
counter=0
for counter in range (original_str):
	counter = counter +  1
num_chars=counter
print(num_chars)

Here's error message:
TypeError: start must be a integer on line 3