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

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

Sentence: 'a man standing on a surf board in the ocean'. Find frequency of the letter 'f'
1

Sentence: 'a young boy holding a soccer ball in his hands'. Find frequency of the letter 'd'

2