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

Example Input: Sentence: 'a girl sits on a surfboard and smiles'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a herd of giraffe walking across a field'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a close up of people at a table with doughnuts'. Find frequency of the letter 'n'
Example Output:
1