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

[Q]: Sentence: 'two persons standing outside a moving sub train'. Find frequency of the letter 'a'
[A]: 3


[Q]: Sentence: 'a red fire hydrant opened and spilling water'. Find frequency of the letter 'd'
[A]: 4


[Q]: Sentence: 'a man in white shirt falling off of a skateboard'. Find frequency of the letter 'a'
[A]:
6