pragma solidity ^0.4.13;

contract SafeMath{
  function safeMul(uint a, uint b) internal returns (uint) {
    uint c = a * b;
    assert(a == 0 || c / a == b);
    return c;
  }

  function safeDiv(uint a, uint b) internal returns (uint) {
    assert(b > 0);
    uint c = a / b;
    assert(a == b * c + a % b);
    return c;
  }
	
	function safeSub(uint a, uint b) internal returns (uint) {
    	assert(b <= a);
    	return a - b;
  }

	function safeAdd(uint a, uint b) internal returns (uint) {
    	uint c = a + b;
    	assert(c >= a);
    	return c;
  }
	function assert(bool assertion) internal {
	    if (!assertion) {
	      revert();
	    }
	}
}

contract Token {

    function totalSupply() constant returns (uint256 supply) {}

    function balanceOf(address _owner) constant returns (uint256 balance) {}

    function transfer(address _to, uint256 _value) returns (bool success) {}

    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {}

    function approve(address _spender, uint256 _value) returns (bool success) {}

    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {}

    event Transfer(address indexed _from, address indexed _to, uint256 _value);
    event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

contract StandardToken is Token , SafeMath{

    function transfer(address _to, uint256 _value) returns (bool success) {
        if (balances[msg.sender] >= _value && _value > 0) {
            balances[msg.sender] = safeSub(balances[msg.sender], _value);
            balances[_to] = safeAdd(balances[_to],_value);
            Transfer(msg.sender, _to, _value);
            return true;
        } else { return false; }
    }

    function transferFrom(address _from, address _to, uint256 _value) returns (bool success) {
         if (balances[_from] >= _value && allowed[_from][msg.sender] >= _value && _value > 0) {
            balances[_to] = safeAdd(balances[_to],_value);
            balances[_from] = safeSub(balances[_from],_value);
            allowed[_from][msg.sender] = safeSub(allowed[_from][msg.sender],_value);
            Transfer(_from, _to, _value);
            return true;
        } else { return false; }
    }

    function balanceOf(address _owner) constant returns (uint256 balance) {
        return balances[_owner];
    }

    function approve(address _spender, uint256 _value) returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }

    function allowance(address _owner, address _spender) constant returns (uint256 remaining) {
      return allowed[_owner][_spender];
    }

    mapping (address => uint256) balances;
    mapping (address => mapping (address => uint256)) allowed;
    uint256 public totalSupply;
}

contract DID is StandardToken {

    function () {
        //if ether is sent to this address, send it back.
        throw;
    }

    
    string public name = "DID";                   
    uint8 public decimals = 18;
    string public symbol = "DID";
    uint256 public INITIAL_SUPPLY = 20000000000000000000000000000;

    function DID() {
        balances[msg.sender] = INITIAL_SUPPLY;
        totalSupply = INITIAL_SUPPLY;
    }

     function approveAndCall(address _spender, uint256 _value, bytes _extraData) returns (bool success) {
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);

        if(!_spender.call(bytes4(bytes32(sha3("receiveApproval(address,uint256,address,bytes)"))), msg.sender, _value, this, _extraData)) { throw; }
        return true;
    }
}