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

Example Input: Sentence: 'two people holding their skies in a snowy resort'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a group of people seated in chairs together and facing the same direction, with one man holding a soccer ball in his hands'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'a lady carrying a birthday cake and two other ladies'. Find frequency of the letter 'i'
Example Output:
3