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

Example Input: Sentence: 'a person riding a camel in front of a truck'. Find frequency of the letter 't'
Example Output: 2

Example Input: Sentence: 'an orange cat sitting on the ground next to a  purse'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'guy pokes through the wall recording the baseball game as a outfielder goes for the ball'. Find frequency of the letter 'l'
Example Output:
7