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

Example Input: Sentence: 'there is a stop sign that has a reflection in a puddle'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'three woman are having a discussion in the kitchen'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a old raggedy train on a rusty train track'. Find frequency of the letter 'y'
Example Output:
2