Solidity unsigned integer less than zero

Question

I was trying to figure out the overflow in solidity, and I noticed that when I subtract one to a uint which was assigned by zero.

The value will become 115792089237316195423570985008687907853269984665640564039457584007913129639935

What does this monster mean?

For example

contract test{
    uint public a = 1;

    function subone(){
        a--;   
    }
}

After I execute twice subone, the value of a becomes that long number. The same thing happened when I have an array and I subtract its length to less than zero.

For example

pragma solidity ^0.4.17; 

contract X{
    uint256[] public array = [1];

    function X(){}

    function popLength() public{
        array.length--;
    }

    function getLength() constant returns(uint256){
        return array.length;
    }
}

Answer 1

That number is 2**256-1. In hexadecimal, it's 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF.