abstract class ArithExpr {
  abstract double eval(Env enf);
  abstract String exprString();
}

class Const extends ArithExpr {

  double value;

  double eval(Env env) { return value;  }
  String exprString() { return Double.toString(value); }
}

 class Var extends ArithExpr {

  String name;

  double eval(Env env) { return env.lookup(name);  }
  String exprString() { return name; }
}

class Sum extends ArithExpr {

  ArithExpr left, right;

  double eval(Env env) { return left.eval(env) + right.eval(env); }
  String exprString() { return "(" + left.exprString() + " + " + right.exprString() + ")"; }
}

class Diff extends ArithExpr {

  ArithExpr left, right;
  
  double eval(Env env) { return left.eval(env) - right.eval(env); }  
  String exprString() { return  "(" + left.exprString() + " - " + right.exprString() + ")"; }
}

class Prod extends ArithExpr {

  ArithExpr left, right;

  double eval(Env env) { return left.eval(env) * right.eval(env); }
  String exprString() { return "(" + left.exprString() + " * " + right.exprString() + ")"; }
}

class Quot extends ArithExpr {

  ArithExpr left, right;

  double eval(Env env) { return left.eval(env) / right.eval(env); }
  String exprString() { return "(" + left.exprString() + " / " + right.exprString() + ")"; }
}

abstract class Env {
  abstract Double lookup(String name);
  Env cons(String vn, double v) { return new ConsEnv(vn, v, this); }
}

class EmptyEnv extends Env {
  static EmptyEnv ONLY = new EmptyEnv();
  private EmptyEnv() { }
  Double lookup(String name) { return null; }
}

class ConsEnv extends Env {
  String varName;
  double value;
  Env rest;
  Double lookup(String name) {
    if (name.equals(varName)) return value;
    else return rest.lookup(name);
  }
}