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

Example Input: Sentence: 'group of cars on the road with the sun setting behind it'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a man sitting inside a green military truck talking to someone'. Find frequency of the letter 'i'
Example Output: 7

Example Input: Sentence: 'two different frame of a woman with a racquet'. Find frequency of the letter 'e'
Example Output:
4