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

Example Input: Sentence: 'a softball player holding the bat on first base'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'hand holding a white calculator inside a store'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a man trekking  through the snowy woods on skis'. Find frequency of the letter 's'
Example Output:
4