code: public float getSequenceTailAverageQuality(int numElements) {
          float quality = -1;
          if (this.size() >= numElements) {
              // Sum the quality of the last 'n' elements of the sequence,
              // and divide the result by 'n' to obtain the average value
              int totalTailQuality = 0;
              for (int i = 1; i <= numElements; i++) {
                  totalTailQuality += this.qualityScoresArray[this.size() - i];
              }
              quality = totalTailQuality / numElements;
          }
          return quality;
      }
docs: Returns the average quality of the last elements of the sequence
  
  @param numElements - Number of elements whose quality will be returned
  @return - Average quality of the last elements of the sequence