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

Ex Input:
Sentence: 'a picture of a toilet and toilet paper in a bathroom'. Find frequency of the letter 'o'

Ex Output:
5

Ex Input:
Sentence: 'an area that has surrounded by a metal fence'. Find frequency of the letter 'o'

Ex Output:
1

Ex Input:
Sentence: 'a football player laying on the ground holding a football while another player is holding him'. Find frequency of the letter 't'

Ex Output:
4