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

Example Input: Sentence: 'the skateboarder has on red and black sneakers and is riding a skateboard'. Find frequency of the letter 'g'
Example Output: 1

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

Example Input: Sentence: 'an elephant is eating some food in a pin'. Find frequency of the letter 'f'
Example Output:
1