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

Ex Input:
Sentence: 'an elephant with broken tusks stands in front of a hill and boulders'. Find frequency of the letter 'n'

Ex Output:
7

Ex Input:
Sentence: 'a young boy standing in front of a counter with dougnuts'. Find frequency of the letter 'o'

Ex Output:
6

Ex Input:
Sentence: 'a group of people standing around a table filled with assorted electronic devices'. Find frequency of the letter 'w'

Ex Output:
1