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

Example Input: Sentence: 'vanity sitting atop a base with two doors and a mirror overhead'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a plastic plastic plate topped with three different type of food'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'several items outlined in white with some orange and red items'. Find frequency of the letter 'n'
Example Output:
4