pragma solidity >=0.6.2;

contract TokenERC20 {
    string public name; 
    string public symbol; 
    uint8 public decimals ;  
    uint256 public totalSupply; 

    mapping (address => uint256) public balanceOf;

    mapping (address => mapping (address => uint256)) public allowance;

    event Transfer(address indexed from, address indexed to, uint256 value);

    constructor (uint256 initialSupply, string memory tokenName, string memory tokenSymbol, uint8 tokenDecimals) public {
        decimals = tokenDecimals;           
        totalSupply = initialSupply * 10 ** uint256(decimals);  
        balanceOf[msg.sender] = totalSupply;                
        name = tokenName;                                   
        symbol = tokenSymbol;                               
    }

    function _transfer(address _from, address _to, uint _value) internal {

        require(balanceOf[_from] >= _value);
  
        require(balanceOf[_to] + _value > balanceOf[_to]);

        uint previousBalances = balanceOf[_from] + balanceOf[_to];
        // Subtract from the sender
        balanceOf[_from] -= _value;
        // Add the same to the recipient
        balanceOf[_to] += _value;
        
        emit Transfer(_from, _to, _value);
        
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }

    function transfer(address _to, uint256 _value) public returns (bool success){
        _transfer(msg.sender, _to, _value);
        return true;
    }


    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);     // Check allowance
        allowance[_from][msg.sender] -= _value;
        _transfer(_from, _to, _value);
        return true;
    }

    function approve(address _spender, uint256 _value) public returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        return true;
    }


}