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

Example Input: Sentence: 'a woman fully dressed as a geisha'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'two zebras feed on a plant through some wire mesh'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a teddy bear sitting on top of a bed with videos and books'. Find frequency of the letter 'd'
Example Output:
5