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

Example Input: Sentence: 'a family out snow skiing on a blustery day'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a tennis player that is running to get the tennis ball'. Find frequency of the letter 't'
Example Output: 7

Example Input: Sentence: 'a plaque on the side of a stone bench'. Find frequency of the letter 'd'
Example Output:
1