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

Ex Input:
Sentence: 'a person on a sidewalk jumping with an umbrella'. Find frequency of the letter 'a'

Ex Output:
5

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: 'two kids are playing baseball in the street'. Find frequency of the letter 't'

Ex Output:
4