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

Example Input: Sentence: 'a group of workers washing and cleaning fruits and vegetables'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a teddy bear is sitting on a heart shaped patio chair outside'. Find frequency of the letter 'i'
Example Output: 6

Example Input: Sentence: 'a man and woman pose for a picture holding umbrellas at night'. Find frequency of the letter 'g'
Example Output:
2