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

Example Input: Sentence: 'a woman is playing an interactive video game'. Find frequency of the letter 't'
Example Output: 2

Example Input: Sentence: 'vanity sitting atop a base with two doors and a mirror overhead'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'two very cute cats laying together near some grass'. Find frequency of the letter 'n'
Example Output:
2