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

Example Input: Sentence: 'supplies and tools are arranged on the floor with a hard hat and bag'. Find frequency of the letter 'n'
Example Output: 4

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

Example Input: Sentence: 'two people holding skis standing at the base of a large snow pile'. Find frequency of the letter 'e'
Example Output:
6