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

Ex Input:
Sentence: 'salad and vegetables with me on top of plate'. Find frequency of the letter 'o'

Ex Output:
3

Ex Input:
Sentence: 'pedestrians pass by a stop sign, some holding surfboards'. Find frequency of the letter 'n'

Ex Output:
3

Ex Input:
Sentence: 'two men styling one woman's hair as she uses a smart phone'. Find frequency of the letter 's'

Ex Output:
7