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

Example Input: Sentence: 'a woman is taking a sip of a drink from a glass'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a flock of sheep grazing in a park'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'the kitchen is filled with retro pictures and appliances'. Find frequency of the letter 'p'
Example Output:
3