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

Example Input: Sentence: 'a group of people are playing soccer on a beach'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a woman is looking at sheep off in a field'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'man and woman holding white controllers in a living room'. Find frequency of the letter 'o'
Example Output:
6