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

Ex Input:
Sentence: 'a lady mixes some things in a blender as people look on'. Find frequency of the letter 'i'

Ex Output:
3

Ex Input:
Sentence: 'group of cars on the road with the sun setting behind it'. Find frequency of the letter 'd'

Ex Output:
2

Ex Input:
Sentence: 'a large semi carrying a green tractor behind it on a flatbed trailer'. Find frequency of the letter 'n'

Ex Output:
4