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

Ex Input:
Sentence: 'a young elephant is touching a bigger elephant with its trunk'. Find frequency of the letter 'i'

Ex Output:
5

Ex Input:
Sentence: 'the skateboarder has on red and black sneakers and is riding a skateboard'. Find frequency of the letter 'g'

Ex Output:
1

Ex Input:
Sentence: 'there is a man playing a game of tennis'. Find frequency of the letter 'm'

Ex Output:
2