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

Example Input: Sentence: 'cars parked outside if a retail store'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a full plate of food that includes rice and broccoli and carrots'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a usb hub with multiple electronics plugged in'. Find frequency of the letter 's'
Example Output:
2