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

Example Input: Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
Example Output: 2

Example Input: Sentence: 'the man is holding a plastic sled with snow in it next to a child'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'two pizzas sit ready to eat on a grill'. Find frequency of the letter 'i'
Example Output:
3