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

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

Ex Output:
3

Ex Input:
Sentence: 'a bowl of granola, dried fruit, and bananas'. Find frequency of the letter 'u'

Ex Output:
1

Ex Input:
Sentence: 'a truck moving along a highway which has a guardrail'. Find frequency of the letter 'a'

Ex Output:
8