HelloWorld.sol

pragma solidity ^0.4.11;

contract HelloWorld {
  uint public balance;
  address public owner;

  function HelloWorld() public {
      owner = msg.sender;
      balance = 0;
  }
  
  function update() public payable isOwner() {
      balance = msg.value;
  }
  
  modifier isOwner() {
      require(msg.sender == owner);
      _;
  }
}