void setup() {
pinMode(2, OUTPUT);
pinMode(3, OUTPUT);
pinMode(4, OUTPUT);
pinMode(5, OUTPUT);
pinMode(6, OUTPUT);
pinMode(7, OUTPUT);


}
int a=50;
void loop() {
a= a+50;
digitalWrite(2, LOW);
digitalWrite(3, LOW);
digitalWrite(4, LOW);
digitalWrite(5, LOW);
digitalWrite(6, LOW);
digitalWrite(7, LOW);


for (int i=2; i<=7; i++){
digitalWrite(i, HIGH);
digitalWrite(i-1, LOW);
delay(a);
digitalWrite(7, LOW);
}

for (int i=6; i>=3; i--){
  digitalWrite(i, HIGH);
  digitalWrite(i+1, LOW);
  delay(a);
  digitalWrite(2, LOW);
  }
}