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

Example Input: Sentence: 'a crowd of people skiing and enjoying the snow'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a couple of boys in a field playing baseball'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a young boy holding a ball behind his back'. Find frequency of the letter 'd'
Example Output:
2