PROGRAMMING:Case conversion

Input a character, if it is uppercase, convert it to lowercase and output; If it is lowercase, convert it to uppercase and output it; If not, output: error
###Input example:
```in
a
```
###Output example:
```out
A
```
###Input example:
```in
B
```
###Output example:
```out
b
```
###Input example:
```in
one
```
###Output example:
```out
error
```







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