Use a dictionary to count the frequency of letters in the input string. Only letters should be counted, not blank spaces, numbers, or punctuation. Upper case should be considered the same as lower case. For example, count_letters ("This is a sentence.") should return {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 1}.

def count_letters (text):

result = {}

# Go through each letter in the text

convert_text = text. lower ()

for letter in convert_text:

if letter. isalpha ():

if letter in result:

result[letter] + = 1

else:

result[letter] = 1

return result

print (count_letters ("AaBbCc"))

# Should be {'a': 2, 'b': 2, 'c': 2}

print (count_letters ("Math is fun! 2+2=4"))

# Should be {'m': 1, 'a': 1, 't': 1, 'h': 1, 'i': 1, 's': 1, 'f': 1, 'u': 1, 'n': 1}

print (count_letters ("This is a sentence."))

# Should be {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 1}