import java.util.Stack;

public class Postfix{
    public static double evaluate (String exp){

	Stack<Double> s = new Stack<Double>();
	String[] tokens = exp.split("\\s+");
	for (int i=0; i<tokens.length; i++){
	    if (tokens[i].equals("+") || tokens[i].equals("-")
		|| tokens[i].equals("*") || tokens[i].equals("/")){

		double op1, op2;

		if (! s.empty())
		    op2 = s.pop();
		else 
		    throw new PostfixException ("invalid postfix expression");

		if (! s.empty())
		    op1 = s.pop();
		else
		    throw new PostfixException ("invalid postfix expression");

		switch (tokens[i].charAt(0)){
		case '+':
		    s.push (op1 + op2);
		    break;
		case '-':
		    s.push (op1 - op2);
		    break;
		case '*':
		    s.push (op1 * op2);
		    break;
		case '/':
		    s.push (op1 / op2);
		    break;
		} // end switch

	    } // end if

	    else{ // if not an operator, assume it is a number
		try{
		    s.push (Double.valueOf(tokens[i]));
		}
		catch (NumberFormatException e){
		    throw new PostfixException("invalid token: "
					       + tokens[i]);
		}
	    } // end else

	} // end for

	double ret;
	if (! s.empty())
	    ret = s.pop();
	else
	    throw new PostfixException ("invalid postfix expression");

	if (s.empty())
	    return ret;
	else 
	    throw new PostfixException ("invalid postfix expression");
	
    }  // end evaluate

}  // end Postfix