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

Example Input: Sentence: 'people seated on a table looking very happy'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'two little girls playing with a bat and a tennis ball'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'a man holding a skateboard in front of a group of people'. Find frequency of the letter 'l'
Example Output:
2