PROGRAMMING:**On the flag of Poland

On the table there are $$n (1 / 10000) $$small flags, part white and part red (the Polish flag consists of white and red). The only allowed operation is to exchange two flags. Please design an algorithm to put all the white flags in front of the red flag with the least switching operation.
####Input format
>The first line is an integer, $$n$$
>The second line is $$n $$characters, w for white flag and R for red flag
####Output format
>Minimum exchange times
####Input sample
```in
six
WRWRRW
```
####Output sample
```out
one
```







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