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

Example Input: Sentence: 'a small wrinkley dog wearing a bow tie'. Find frequency of the letter 'e'
Example Output: 3

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: 'a fire hydrant is beside a curb, bushes and a tree'. Find frequency of the letter 'i'
Example Output:
3