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

Example Input: Sentence: 'a log in the water with three birds perched on it'. Find frequency of the letter 'r'
Example Output: 4

Example Input: Sentence: 'men standing in the grass with a frisbee'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'man on pink surf board surfing on the waves'. Find frequency of the letter 'p'
Example Output:
1