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

Ex Input:
Sentence: 'three woman are having a discussion in the kitchen'. Find frequency of the letter 'n'

Ex Output:
5

Ex Input:
Sentence: 'a man riding on the back of a white horse next to other horses'. Find frequency of the letter 'o'

Ex Output:
6

Ex Input:
Sentence: 'a tennis player getting ready to hit a serve'. Find frequency of the letter 't'

Ex Output:
5