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

Example Input: Sentence: 'a blue tripocal bird is perched on a wooden fence'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a man hitting a tennis ball with a tennis racquet'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a man with a tie and a work badge'. Find frequency of the letter 'a'
Example Output:
6