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

Example Input: Sentence: 'a castle like bed room that has the era of templar knight similarities'. Find frequency of the letter 'm'
Example Output: 3

Example Input: Sentence: 'a person is riding in a cart pulled by a horse'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a close up of a dog sitting on a chair'. Find frequency of the letter 'n'
Example Output:
2