docs: public int calculate(){
  		if(op<0|num1<0|num2<0)
  			return -1;
  		switch(op){
  			case 0:
  				ans=num1+num2;
  				break;
  			case 1:
  				ans=num1-num2;
  				break;
  			case 2:
  				ans=num1*num2;
  				break;
  			case 3:
  				ans=num1/num2;
  				break;
  			case 4:
  				ans=num2/num1;
  				break;
  			default:
  				return -1;
  		}
  		return 0;
  	}