There is a maximum capacity an integer can store. And when you go over that limit, it will loop back to the negative side. For int, it is 2147483647

int x = int.MaxValue;                //MaxValue is 2147483647
x = unchecked(x + 1);                //make operation explicitly unchecked so that the example also works when the check for arithmetic overflow/underflow is enabled in the project settings 
Console.WriteLine(x);                //Will print -2147483648
Console.WriteLine(int.MinValue);     //Same as Min value

For any integers out of this range use namespace System.Numerics which has datatype BigInteger. Check below link for more information https://msdn.microsoft.com/en-us/library/system.numerics.biginteger(v=vs.110).aspx

Overflow also happens during the operation. In the following example, x is an int, 1 is an int by default. Therefore addition is an int addition. And the result will be an int. And it will overflow.

int x = int.MaxValue;               //MaxValue is 2147483647
long y = x + 1;                     //It will be overflown
Console.WriteLine(y);               //Will print -2147483648
Console.WriteLine(int.MinValue);    //Same as Min value

You can prevent that by using 1L. Now 1 will be a long and addition will be a long addition

int x = int.MaxValue;               //MaxValue is 2147483647
long y = x + 1L;                    //It will be OK
Console.WriteLine(y);               //Will print 2147483648

There is overflow in the following code

int x = int.MaxValue;
Console.WriteLine(x + x + 1L);  //prints -1

Whereas in the following code there is no overflow

int x = int.MaxValue;
Console.WriteLine(x + 1L + x);  //prints 4294967295

This is due to the left-to-right ordering of the operations. In the first code fragment x + x overflows and after that it becomes a long. On the other hand x + 1L becomes long and after that x is added to this value.