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

Ex Input:
Sentence: 'a laptop computer with pictures of giraffes on the homescreen'. Find frequency of the letter 'o'

Ex Output:
5

Ex Input:
Sentence: 'a train sits on a railroad track as a metal awning covers it'. Find frequency of the letter 'l'

Ex Output:
2

Ex Input:
Sentence: 'colorful shapes and abstract art is displayed on a wall'. Find frequency of the letter 'i'

Ex Output:
2