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

Ex Input:
Sentence: 'a woman is looking at sheep off in a field'. Find frequency of the letter 'm'

Ex Output:
1

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

Ex Output:
3

Ex Input:
Sentence: 'a pizza sits on a plate atop of a wooden table'. Find frequency of the letter 't'

Ex Output:
4