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

Example Input: Sentence: 'young boy flying kite in large field at outdoor event'. Find frequency of the letter 'y'
Example Output: 3

Example Input: Sentence: 'a closeup view of a bathroom sink, that is very modern'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a display with different types of live sheep'. Find frequency of the letter 'p'
Example Output:
3