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

Ex Input:
Sentence: 'a crowd of fans watch soccer players from stadium seating'. Find frequency of the letter 'f'

Ex Output:
3

Ex Input:
Sentence: 'a person dressed all in black snowboarding down a slope'. Find frequency of the letter 'e'

Ex Output:
4

Ex Input:
Sentence: 'salad and vegetables with me on top of plate'. Find frequency of the letter 'o'

Ex Output:
3