import java.util.*;

public class Evaluator extends ExpressionVisitor {

  private Stack<Integer> values = new Stack<Integer>();

  public void visitIntExp(IntExp e) {
    values.push( e.getValue() );
  }

  public void visitAddExp(AddExp e) {
    e.getE1().accept(this);
    int x1 = values.pop();
    e.getE2().accept(this);
    int x2 = values.pop();
    values.push(x1+x2);
  }

  public int finalValue() {
    return values.pop();
  }

}