int a=2;
int b=3;
void setup() {
  pinMode(a, OUTPUT);
  pinMode(b, OUTPUT);
}

void loop() {
  digitalWrite(a, HIGH);
  delay(1000);
  digitalWrite(a, LOW); 
  delay(1000);
  digitalWrite(b, HIGH);
  delay(1000);
  digitalWrite(b, LOW);
  delay(1000);
}