define Ap 20
define An 21
define Bp 19
define Bn 18

void setup() {
Pinmode(Ap, OUTPUT);
Pinmode(An, OUTPUT);
Pinmode(Bp, OUTPUT);
Pinmode(Bn, OUTPUT);
}

void loop(){
delay (10);
digitalWrite(Bn, LOW); digitalWrite(Ap, HIGH);
delay (10);
digitalWrite(Ap, LOW); digitalWrite(Bp, HIGH);
delay (10);
digitalWrite(Bp, LOW); digitalWrite(An, HIGH);
delay (10);
digitalWrite(An, LOW); digitalWrite(Bn, HIGH);
}