void setup() {
  // put your setup code here, to run once:
  pinMode (13, OUTPUT);
  pinMode (12, OUTPUT);
  pinMode (11, OUTPUT);
  pinMode (10, OUTPUT);
  pinMode (9, OUTPUT);
  pinMode (8, OUTPUT);
  
}

void loop() {
  // put your main code here, to run repeatedly:
  stepAhead () ;
  stepBack () ;
  stepAhead () ;
  stepBack () ;
  stepAhead () ;
  stepBack () ;
  stepAhead () ;
  stepBack () ;
  delay (1000);
}

void stepAhead () {
  //step ahead
  digitalWrite (12, HIGH);
  digitalWrite (13, LOW);
  digitalWrite (8, HIGH);
  digitalWrite (9, LOW);
  //turn on the motors 
  digitalWrite (11, HIGH);
  digitalWrite (10, HIGH);
  delay (100);
  //turn off the motors
  digitalWrite (11, LOW);
  digitalWrite (10, LOW);
  delay (430);
}

void stepBack () {
   //step back
  digitalWrite (12, LOW);
  digitalWrite (13, HIGH);
  digitalWrite (8, LOW);
  digitalWrite (9, HIGH);
  //turn on the motors 
  digitalWrite (11, HIGH);
  digitalWrite (10, HIGH);
  delay (100);
  //turn off the motors
  digitalWrite (11, LOW);
  digitalWrite (10, LOW);
  delay (430);
}