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

Example Input: Sentence: 'several zebras  and a giraffe in an enclosure'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a bird is standing on the ledge of the building'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'paper plates hold fruit salad and two kinds of cake'. Find frequency of the letter 'd'
Example Output:
4