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

Ex Input:
Sentence: 'two people and two children walking in a building'. Find frequency of the letter 'd'

Ex Output:
3

Ex Input:
Sentence: 'two giraffes standing next to some trees'. Find frequency of the letter 'f'

Ex Output:
2

Ex Input:
Sentence: 'airplanes are shown both at the gate and on the runway'. Find frequency of the letter 'a'

Ex Output:
7