int a=13;
int b=12;
int c=11;
int d=10;
int t=50;

void setup() {
  // put your setup code here, to run once:
pinMode(a,OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:
digitalWrite(a, HIGH);
delay(t);
digitalWrite(a, LOW);
delay(t);
digitalWrite(b,HIGH);
delay(t);
digitalWrite(b, LOW);
delay(t);
digitalWrite(c, HIGH);
delay(t);
digitalWrite(c, LOW);
delay(t);
digitalWrite(d, HIGH);
delay(t);
digitalWrite(d, LOW);
delay(t);

}