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

Example Input: Sentence: 'a woman at a table checking her cell phone'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'vanity sitting atop a base with two doors and a mirror overhead'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'there are a man and a horse inside of the barn'. Find frequency of the letter 'i'
Example Output:
2