void setup()
{
  Serial.begin( 115200 );
}

void loop()
{
  const  uint8_t  number_of_readings = 10;
  const  uint32_t interval1          = 100; 
  const  uint32_t interval2          = 200;

  const  uint32_t timestamp          = millis();
  static uint32_t timer_timestamp    = timestamp;
  static uint32_t timer_interval     = 0;

  if ( timestamp - timer_timestamp >= timer_interval )
  {
    timer_timestamp = timestamp;
    static bool toggle = false; 

    static uint32_t readings_sum = 0;
    readings_sum += analogRead( A0 );

    static uint8_t readings_counter = 0;
    if ( ++readings_counter == number_of_readings )
    {
      readings_counter = 0;
      toggle = false;

      const uint32_t readings_average = readings_sum / number_of_readings;
      Serial.print( "readings_average = " );
      Serial.println( readings_average );
      readings_sum = 0;
    }

    toggle = !toggle;
    timer_interval = ( toggle == true ) ? interval1 : interval2;
  }
}