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

Example Input: Sentence: 'oranges are sitting in a bowl on a table'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'soccer players on opposite teams fight for control over a ball'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'three elephants walking in a field near one another'. Find frequency of the letter 'd'
Example Output:
1