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

Sentence: 'tow birds flying by a long necked crane'. Find frequency of the letter 'f'
1

Sentence: 'a black cat laying down on a bed'. Find frequency of the letter 'y'
1

Sentence: 'a view of mountains and plains as seen from an airplane window'. Find frequency of the letter 's'
4