static int EvaluateExpression(Expression ex) {
	if (ex is BinaryExpression) {
		BinaryExpression bEx = (BinaryExpression) ex;
		if (ex is DivideExpression) {
			return checked(EvaluateExpression(bEx.Op0) / EvaluateExpression(bEx.Op1));
		} else if (ex is MinusExpression) {
			return checked(EvaluateExpression(bEx.Op0) - EvaluateExpression(bEx.Op1));
		} else if (ex is MultiplyExpression) {
			return checked(EvaluateExpression(bEx.Op0) * EvaluateExpression(bEx.Op1));
		} else if (ex is PlusExpression) {
			return checked(EvaluateExpression(bEx.Op0) + EvaluateExpression(bEx.Op1));
		} else {
			throw new NotSupportedException();
		}
	} else if (ex is ConstantExpression) {
		return ((ConstantExpression) ex).Value;
	} else {
		UnaryMinusExpression uEx = (UnaryMinusExpression) ex;
		return -EvaluateExpression(uEx.Op);
	}
}