contract SafeMath {
  uint256 constant private MAX_UINT256 =
    0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF;

  /**
   * Add two uint256 values, throw in case of overflow.
   *
   * @param x first value to add
   * @param y second value to add
   * @return x + y
   */
  function safeAdd (uint256 x, uint256 y)
  pure internal
  returns (uint256 z) {
    assert (x <= MAX_UINT256 - y);
    return x + y;
  }

  /**
   * Subtract one uint256 value from another, throw in case of underflow.
   *
   * @param x value to subtract from
   * @param y value to subtract
   * @return x - y
   */
  function safeSub (uint256 x, uint256 y)
  pure internal
  returns (uint256 z) {
    assert (x >= y);
    return x - y;
  }

  /**
   * Multiply two uint256 values, throw in case of overflow.
   *
   * @param x first value to multiply
   * @param y second value to multiply
   * @return x * y
   */
  function safeMul (uint256 x, uint256 y)
  pure internal
  returns (uint256 z) {
    if (y == 0) return 0; // Prevent division by zero at the next line
    assert (x <= MAX_UINT256 / y);
    return x * y;
  }
}
/**
 * Provides methods to safely add, subtract and multiply uint256 numbers.
 */
function safeAdd (uint256 x, uint256 y)
  pure internal
  returns (uint256 z) {
    assert (x <= MAX_UINT256 - y);
    return x + y;
  }
/**
 * Add two uint256 values, throw in case of overflow.
 *
 * @param x first value to add
 * @param y second value to add
 * @return x + y
 */