public static double sd(SlidingWindow left, SlidingWindow right) {
    int n = left.maxSize + right.maxSize;
    double sumOfSquares = (left.sumOfSquares() + right.sumOfSquares());
    double sum = left.sum() + right.sum();

    return Math.sqrt((n * (sumOfSquares) - (sum * sum)) / (n * (n - 1)));
  }
 public static double mean(SlidingWindow left, SlidingWindow right) {
   return (left.mean() + right.mean()) / 2;
 }