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

Example Input: Sentence: 'a wedding cake with pink roses on a silver plate'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'older man getting ready to swing his bat at a ball on a stand in front of a group of viewers'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'a woman swinging a tennis racquet towards a tennis ball'. Find frequency of the letter 'e'
Example Output:
3