While loop "Sum of numbers"

I started learning Python, sometimes terrible stupidities attack.
A number of tasks with this loop, it seems, are solved and understandable,
but I can’t understand how to formulate the solution?

Determine the sum of all elements of the sequence up to 0.
In this and the following tasks, the numbers after zero are not considered. They are taken into account.
def was not covered in this course, I don’t know what it is.
i.e., the problem must be solved within the framework of the studied material:

1. input-output, arithmetic operations
2. conditional instruction
3. operations with integers and real numbers
4. for loop
5. strings
6. while loop

That's just