void setup() {
Serial.begin(9600);
for (int led = 2; led<12;led = led+1){
  pinMode(led, OUTPUT);
}

}

void loop() {
for (int pin = 2; pin<12; pin = pin+1){
  digitalWrite(pin, HIGH);
  delay(100);
  digitalWrite(pin, LOW);
  delay(100);
}
for (int pin = 12; pin>2; pin = pin-1){
  digitalWrite(pin, HIGH);
  delay(100);
  digitalWrite(pin, LOW);
  delay(100);
}
}