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 group of workers washing and cleaning fruits and vegetables'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'red buses sitting in a line in front of a city building'. Find frequency of the letter 'n'
Example Output:
6