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

Example Input: Sentence: 'a bird is standing on the ledge of the building'. Find frequency of the letter 't'
Example Output: 3

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 girl is eating a slice of pizza'. Find frequency of the letter 'a'
Example Output:
4