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

Ex Input:
Sentence: 'a man playing baseball on a television set'. Find frequency of the letter 'l'

Ex Output:
4

Ex Input:
Sentence: 'a truck with a camper sits parked in some vegetation'. Find frequency of the letter 's'

Ex Output:
3

Ex Input:
Sentence: 'a beautiful young woman hitting a tennis ball with a racquet'. Find frequency of the letter 'y'

Ex Output:
1