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

Example Input: Sentence: 'a woman at a table checking her cell phone'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a closeup view of a bathroom sink, that is very modern'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a person standing on skis with a backpack in front of them'. Find frequency of the letter 'o'
Example Output:
4