interface ERC20 {
    function totalSupply() public view returns (uint supply);
    function balanceOf(address _owner) public view returns (uint balance);
    function transfer(address _to, uint _value) public; // Some ERC20 doesn't have return
    function transferFrom(address _from, address _to, uint _value) public; // Some ERC20 doesn't have return
    function approve(address _spender, uint _value) public; // Some ERC20 doesn't have return
    function allowance(address _owner, address _spender) public view returns (uint remaining);
    function decimals() public view returns(uint digits);
    event Approval(address indexed _owner, address indexed _spender, uint _value);
}
function allowance(address _owner, address _spender) public view returns (uint remaining);
// Some ERC20 doesn't have return