In this task, you need to count the number of times the given letter appears in the given sentence.

Example Input: Sentence: 'two people under an umbrella while walking down a sidewalk'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a man in a sweatshirt is taking his own picture'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a group of people are playing soccer on a beach'. Find frequency of the letter 'l'
Example Output:
2