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

Example Input: Sentence: 'man in air skateboarding on street with building'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a surfer is in the middle of an ocean wave'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'the cows are grazing on the grassy field'. Find frequency of the letter 'r'
Example Output:
3