library SafeMath {

  /**
  * @dev Subtracts two numbers, reverts on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b <= a);
    uint256 c = a - b;

    return c;
  }

  /**
  * @dev Adds two numbers, reverts on overflow.
  */
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    require(c >= a);

    return c;
  }

}
/**
 * @title SafeMath
 * @dev Math operations with safety checks that revert on error
 */
function sub(uint256 a, uint256 b) internal pure returns (uint256) {
  require(b <= a);
  uint256 c = a - b;

  return c;
}

/**
 * @dev Subtracts two numbers, reverts on overflow (i.e. if subtrahend is greater than minuend).
 */