overflow of the integer type in C#

It behaves as towards the circle.

You choose one place to start (it is black line here). Then you are going to make a full circle and then you will reach maximum of its size.

Now, instead of having a problem you will start the same circle again(blue line are those 2 numbers) and that is the reason why you move to negative numbers after reaching a maximum of positive.

This can cause many problems, so it is the best to make some if-else(or something else) to secure your program.

Here is your example.
When we add something this happens: 2,147,483,647 -> -2,147,483,648 -> -2,147,483,647 ...

If you have used unsigned int and tried to decrease them(4,3,2,1,0 ..) then instead of moving to negative numbers you would move to a maximum value which is 4,294,967,295

Edit:

It is runtime behaviour

Credits to Klaus (look comments).