The current implementation does not contain guard and it works well,
but it may lead to problems in the future.
Signed-off-by: Sergii Piatakov <sergii.piatakov@globallogic.com>
Introduce the `Calculator` class with single member-function which
performs an addition.
Signed-off-by: Sergii Piatakov <sergii.piatakov@globallogic.com>