BigDecimal two = BigDecimal.valueOf(2); // Android doesn't have BigDecimal.TWO yet
    BigDecimal bigMean = new BigDecimal(bigX.add(bigY)).divide(two, RoundingMode.FLOOR);
    return bigMean.longValueExact();
  }

  private static boolean fitsInLong(BigInteger big) {
    return big.bitLength() <= 63;
  }

  private static final BigInteger MAX_LONG = BigInteger.valueOf(Long.MAX_VALUE);