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

Example Input: Sentence: 'a gray colored pigeon that is perched on a railing outside of a building, with other buildings in the background'. Find frequency of the letter 'e'
Example Output: 7

Example Input: Sentence: 'a red fire hydrant opened and spilling water'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'a tall street light, camera, street sign combined into one'. Find frequency of the letter 's'
Example Output:
3

