int pinsCount=8;
int pins[]={2,4,5,18,19,13,21,12};

void setup(){
  for (int i=0; i<pinsCount ; i=i+1){
    pinMode(pins[i], OUTPUT);
  }

}
void loop(){
  for(int i=0;i<pinsCount/2;i=i+1){
    digitalWrite(pins[i],HIGH);
    digitalWrite(pins[pinsCount-1-i],HIGH);
    delay(300);
    digitalWrite(pins[i],LOW);
    digitalWrite(pins[pinsCount-1-i],LOW);
    
  }
  for(int i=(pinsCount-1)/2;i>0;i=i-1){
    digitalWrite(pins[i],HIGH);
    digitalWrite(pins[pinsCount-1-i],HIGH);
    delay(300);
    digitalWrite(pins[i],LOW);
    digitalWrite(pins[pinsCount-1-i],LOW);
  }

}