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

Sentence: 'a white police vehicle parked on a paved street'. Find frequency of the letter 'a'
4

Sentence: 'soccer players on opposite teams fight for control over a ball'. Find frequency of the letter 't'
4

Sentence: 'a guy on a snowboard skiing down a slop'. Find frequency of the letter 'l'

1