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

Ex Input:
Sentence: 'a little boy that has a toothbrush in his mouth'. Find frequency of the letter 'r'

Ex Output:
1

Ex Input:
Sentence: 'silhouette of person playing frisbee with moutain and sunset'. Find frequency of the letter 'i'

Ex Output:
5

Ex Input:
Sentence: 'the street lights are shining bright on the dark street'. Find frequency of the letter 'e'

Ex Output:
7