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: 'a white paper plate with a slice of pizza on it'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'the kitchen area is clean and ready to use'. Find frequency of the letter 't'
Example Output:
3