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

Example Input: Sentence: 'a woman throwing a frisbee on a field'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'man riding a skate board down the little board way'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'three elephants walk up a dirt trail from the water'. Find frequency of the letter 'l'
Example Output:
3