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

Example Input: Sentence: 'a man in costume talking on a cell phone'. Find frequency of the letter 'm'
Example Output: 2

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: 'two boys in a field with a frisbee'. Find frequency of the letter 'i'
Example Output:
4