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

Example Input: Sentence: 'a woman standing in front of a bathroom sink in front of a mirror'. Find frequency of the letter 'o'
Example Output: 8

Example Input: Sentence: 'a gray airplane with metal petals on the wings takes off from an airport'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a white sink sitting in a blue area under a brick wall'. Find frequency of the letter 'a'
Example Output:
6