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

Example Input: Sentence: 'a dog laying on the back of a boat in the water'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a group of cars and house on truck passing in a street'. Find frequency of the letter 'i'
Example Output: 2

Example Input: Sentence: 'a black and white photo with two males on cellphones'. Find frequency of the letter 'm'
Example Output:
1