library SafeMath {
  /**
   * SafeMath mul function
   * @dev function for safe multiply
   **/
  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a * b;
    assert(a == 0 || c / a == b);
    return c;
  }
  
  /**
   * SafeMath div funciotn
   * @dev function for safe devide
   **/
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a / b;
    return c;
  }
  
  /**
   * SafeMath sub function
   * @dev function for safe subtraction
   **/
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }
  
  /**
   * SafeMath add fuction 
   * @dev function for safe addition
   **/
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    assert(c >= a);
    return c;
  }
}
/**
 * @title SafeMath
 * @dev Math operations with safety checks that throw on error
 * @notice https://github.com/OpenZeppelin/zeppelin-solidity/blob/master/contracts/math/SafeMath.sol
 */
function mul(uint256 a, uint256 b) internal pure returns (uint256) {
  uint256 c = a * b;
  assert(a == 0 || c / a == b);
  return c;
}

/**
 * SafeMath mul function
 * @dev function for safe multiply
 **/