% ./a.out
Input data by Reverse Polish Notation: 12+34+*

|         |
+---------+
|    1    |
+---------+

|         |
+---------+
|    2    |
+---------+
|    1    |
+---------+

|         |
+---------+
|    3    |
+---------+

|         |
+---------+
|    3    |
+---------+
|    3    |
+---------+

|         |
+---------+
|    4    |
+---------+
|    3    |
+---------+
|    3    |
+---------+

|         |
+---------+
|    7    |
+---------+
|    3    |
+---------+

|         |
+---------+
|   21    |
+---------+
Answer: 21