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

Example Input: Sentence: 'many different species of sheep in display at a museum'. Find frequency of the letter 'h'
Example Output: 1

Example Input: Sentence: 'cars parked outside if a retail store'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'two hot dogs served on a plate with tortilla chips'. Find frequency of the letter 'o'
Example Output:
5