Add The Last Two Values

[Vilmar]

Hi!
I have values ​​up to month four and I need a measurement that adds the last two months to the end of the year.

Exemple:

Mes 5 = 15 (8+7);
Mes 6 = 23 (15+8);

Mes 7 = 38 (23+15);

[Vilmar]

Any help please