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

Example Input: Sentence: 'a car accident has slowed traffic as a clean up crew works'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'tow birds flying by a long necked crane'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a dog is lying on a sidewalk in front of a building'. Find frequency of the letter 'l'
Example Output:
3