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

Ex Input:
Sentence: 'a red fire hydrant opened and spilling water'. Find frequency of the letter 'd'

Ex Output:
4

Ex Input:
Sentence: 'a man in costume talking on a cell phone'. Find frequency of the letter 'm'

Ex Output:
2

Ex Input:
Sentence: 'a street pole with signs and lights on it'. Find frequency of the letter 'i'

Ex Output:
4