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

Example Input: Sentence: 'men are riding on motorcycles with a man riding in their carriers'. Find frequency of the letter 'r'
Example Output: 8

Example Input: Sentence: 'a man in a suit makes the first slice into a cake'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a man runs to hit a tennis ball'. Find frequency of the letter 'h'
Example Output:
1