Super basic token

ERC20Token.sol

pragma solidity ^0.4.24;

contract ERC20Token {

    uint public supply = 1000000;
    uint public price = 0.001 ether;
    string public name = "Hackathon Token";
    string public symbol = "HKT";
    uint public decimals = 18;
    
    address public owner;

    mapping(address => uint256) balances;
    
    constructor() public {
        owner = msg.sender;
        balances[msg.sender] = supply * 10 ** decimals;
    }
    
    function totalSupply() public constant returns (uint) {
        return supply;
    }
    
    function balanceOf(address tokenOwner) public view returns (uint balance) {
       return balances[tokenOwner];  
    }
    
    function transfer(address to, uint tokens) public returns (bool success) {
        // decrement the balance from msg.sender
        // increment token count for to
        uint totalTokens = tokens * 10 ** decimals;
        require(totalTokens <= balances[msg.sender]);
        require(to != address(0));
        balances[msg.sender] = balances[msg.sender] - totalTokens;
        balances[to] = balances[to] + totalTokens;
        return true;
    }

    function purchase() public payable {
        uint tokens = msg.value/price;
        uint totalTokens = tokens * 10 ** decimals;
        require(totalTokens <= balances[owner]);
        balances[owner] = balances[owner] - totalTokens;
        balances[msg.sender] = balances[msg.sender] + totalTokens;
    }
}