PROGRAMMING:Find the value of prefix expression
Arithmetic expression has prefix representation, infix representation and suffix representation. Prefix expression means that binary operators precede two operands. For example, the prefix expression of '2 + 3 * (7-4) + 8 / 4' is' + + 2 * 3 - 7 4 / 8 4 '. Please design a program to calculate the result value of the prefix expression.
###Input format:
Input a prefix expression with no more than 30 characters in one line, which only contains' + ',' - ',' * ',' / ', and operands. Different objects (operands and symbols) are separated by spaces.
###Output format:
Output the operation result of prefix expression, with 1 decimal place reserved, or the error message 'error'.
###Input example:
```in
+ + 2 * 3 - 7 4 / 8 4
```
###Output example:
```out
thirteen
```
answer：If there is no answer, please comment
###Input format:
Input a prefix expression with no more than 30 characters in one line, which only contains' + ',' - ',' * ',' / ', and operands. Different objects (operands and symbols) are separated by spaces.
###Output format:
Output the operation result of prefix expression, with 1 decimal place reserved, or the error message 'error'.
###Input example:
```in
+ + 2 * 3 - 7 4 / 8 4
```
###Output example:
```out
thirteen
```
answer：If there is no answer, please comment