About the terminology:
1) feature is not a "problem". It would rather be a solution. In software programming, a feature is a something that your program does to solve a problem. A feature could be the ability to compute the area of a rectangle.
2) a scenario is a description of the usage of your feature. Like an example. Like a test case, but usually in a more human-readable form.
3) a story (in Agile terminology, in which BDD stands) is a way to describe the a need/problem. Your problem ("as a student...") is presented as a story. This story will lead to a new feature in your soft. This new feature will be tested by scenarios.
About your scenarios.
Yours are not correct.
There is no way to know that if you have 1 nb you should compute an area.
You should have several scenarios, like
Given I send the number 2
When I launch the computation
Then I get the result 12,56
Given I send the number 2 and 3
When I launch the computation
Then I get the result 10
Given I send the number -4
When I launch the computation
Then I get the result error
Given I send the number 1 3 7
When I launch the computation
Then I get the result error