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

Example Input: Sentence: 'a traffic signal by a wire filled with some birds'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a group of men standing around a table cutting a cake'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'two giraffes roaming with a van of people looking on'. Find frequency of the letter 'v'
Example Output:
1