library SafeMath {

  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    if (a == 0) {
      return 0;
    }

    c = a * b;
    assert(c / a == b);
    return c;
  }
}
function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
  // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
  // benefit is lost if 'b' is also tested.
  // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
  if (a == 0) {
    return 0;
  }

  c = a * b;
  assert(c / a == b);
  return c;
}

/**
 * @dev Multiplies two numbers, throws on overflow.
 */