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

[Q]: Sentence: 'an area that has surrounded by a metal fence'. Find frequency of the letter 'o'
[A]: 1

[Q]: Sentence: 'a black computer with a screen reading "press any key to begin."'. Find frequency of the letter 'a'
[A]: 5

[Q]: Sentence: 'lady playing racket ball on a green court, with blue wall'. Find frequency of the letter 'l'
[A]:
7