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

Example Input: Sentence: 'a child sitting a table with a pizza in front of him'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a couple of buildings that has a bus out front'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a group of boys in uniforms playing baseball in a field'. Find frequency of the letter 'n'
Example Output:
4