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

Ex Input:
Sentence: 'a man and several white red and blue airplanes'. Find frequency of the letter 'u'

Ex Output:
1

Ex Input:
Sentence: 'a blue tripocal bird is perched on a wooden fence'. Find frequency of the letter 'd'

Ex Output:
3

Ex Input:
Sentence: 'a woman holding up a smart phone and a little girl'. Find frequency of the letter 'e'

Ex Output:
2