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

Example Input: Sentence: 'a toasted sub sandwich is next to a salad'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a person is holding a chocolate doughnut with rainbow sprinkles'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a little black bird standing on top of a pole'. Find frequency of the letter 'i'
Example Output:
3