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

Ex Input:
Sentence: 'a traffic signal by a wire filled with some birds'. Find frequency of the letter 'e'

Ex Output:
3

Ex Input:
Sentence: 'a double parking meter is decorated with a picture of a girl with wings on her feet'. Find frequency of the letter 'r'

Ex Output:
6

Ex Input:
Sentence: 'a display with giant tooth brushes in a building'. Find frequency of the letter 'p'

Ex Output:
1