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

Example Input: Sentence: 'a full plate of food that includes rice and broccoli and carrots'. Find frequency of the letter 's'
Example Output: 2

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: 'several people standing outside of a red train'. Find frequency of the letter 'e'
Example Output:
6