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

Example Input: Sentence: 'a herd of giraffe walking across a field'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a large motorcycle that has a man dressed in old clothing next to it'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a sheep stands beside the back of a tractor in a field'. Find frequency of the letter 'l'
Example Output:
1