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

Example Input: Sentence: 'a few giraffes standing in a field looking around'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a man riding a skateboard up the side of a ramp'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a yellow banana in a rather dark location'. Find frequency of the letter 'l'
Example Output:
3