int a = 0;
int b = 0;

void setup() {
  pinMode(26, OUTPUT);  // led 3 (and)
  pinMode(27, OUTPUT);  // led 2 (or)
  pinMode(28, OUTPUT);  // led 1 (xor)

  pinMode(6, OUTPUT);   // led a
  pinMode(10, OUTPUT);  // led b

  pinMode(3, INPUT);    // btn A
  pinMode(4, INPUT);    // btn B

}

void loop() {
  if(digitalRead(3)==HIGH){
    if (a == 1){
      a = 0;
    }else{
      a = 1;
    }
  }

  if(digitalRead(4)==HIGH){
    if (b == 1 ){
      b = 0;
    }else{
      b = 1;
    }
  }

  // led A
  if (a == 1){
    digitalWrite(6, HIGH);
  }else{
    digitalWrite(6, LOW);
  }

  // led B
  if (b == 1){
    digitalWrite(10, HIGH);
  }else{
    digitalWrite(10, LOW);
  }

  // AND
  if (a == 1 && b == 1){
    digitalWrite(26, HIGH);
  }else{
    digitalWrite(26, LOW);
  }

  // OR
  if (a == 0 && b == 0){
    digitalWrite(27, LOW);
  }else{
    digitalWrite(27, HIGH);  
  }

  // XOR
  if (a == b){
    digitalWrite(28, LOW);  
  }else{
    digitalWrite(28, HIGH);  
  }

}