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

Ex Input:
Sentence: 'a gray elephant is in a grassy field'. Find frequency of the letter 'e'

Ex Output:
3

Ex Input:
Sentence: 'a bird outside standing on a table looking up'. Find frequency of the letter 'b'

Ex Output:
2

Ex Input:
Sentence: 'a double decker bus is sitting in a lane in europe'. Find frequency of the letter 'n'

Ex Output:
4