//Exercise 1.3
int led1 = D2;
int led2 = D0;
//define 2 led

void setup() {
    
    pinMode( led1, OUTPUT ); 
    pinMode( led2, OUTPUT );
}

void loop() {

    digitalWrite( led1, HIGH );
    digitalWrite( led2, LOW);
    delay( 3000 );

    
    digitalWrite( led1, LOW );
    digitalWrite( led2, HIGH );
    delay( 3000 );
    //the 2 led work in opposite way
}