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

Example Input: Sentence: 'a giraffe sticking it's tongue out near a window'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a woman swinging a tennis racquet towards a tennis ball'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'two woman on the beach holding their surfboards'. Find frequency of the letter 'r'
Example Output:
3