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

Example Input: Sentence: 'a scene at night; a plane alone on a runway with lit-up city buildings on the horizon'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'a yellow fire hydrant in the forefront of a roadway'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a woman leaning on a building talking on her cell phone'. Find frequency of the letter 'a'
Example Output:
5