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

Example Input: Sentence: 'four men standing around two pizzas in a conference room'. Find frequency of the letter 't'
Example Output: 2

Example Input: Sentence: 'a large cooked pizza on a dining table'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a white plate topped with squid, greens and oranges'. Find frequency of the letter 'p'
Example Output:
3