long runningAverage(int M)
{
  static int LM[10];      // LastMeasurements
  static byte index = 0;
  static long sum = 0;
  static byte count = 0;

  // keep sum updated to improve speed.
  sum -= LM[index];
  LM[index] = M;
  sum += LM[index];
  index = index % LMSIZE;
  if (count < LMSIZE) count++;

  return sum / count;
}