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

Example Input: Sentence: 'two men are playing a video game as another sits watching'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'people playing wii bowling in a modern library'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a white toilet has a fancy control panel'. Find frequency of the letter 'l'
Example Output:
3