public void exec(){   						
	        boolean nextStep = true;               

	        do {
	        	try{ 
	
	        		inpFirstArg();
	        		
	        		inpOperation();
	           	 	
	           	 	res = unOp(inpOperation);
	           	 	if (res != null){	
	           	 			out.println(res);   
	           	 	}else{
	           	 			inpSecondArg();
	           	 			res = binOp(inpOperation);
	           	 			if(res != null){
	           	 				out.println(res);	
	           	 			} else if ((inpOperation>7)||(inpOperation<0)){
	           	 				out.println("There is no such operation!");
	           	 			}
	           	 	}	
	           	 	
	           	 	inpWhatToDo();
	           		if (whatDo == 2) nextStep = false;
	        	 }catch(Exception e){
				    out.println("ERROR!!! Enter a number.");
	        	 }
	        } while (nextStep);
	    }