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

Example Input: Sentence: 'three elephants walk up a dirt trail from the water'. Find frequency of the letter 'l'
Example Output: 3

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: 'a galloping giraffe in a fenced in area at a zoo'. Find frequency of the letter 'a'
Example Output:
8