Problem:
Score : 400 points

Problem Statement

An altar enshrines N stones arranged in a row from left to right. The color of the i-th stone from the left (1 \leq i \leq N) is given to you as a character c_i; R stands for red and W stands for white.

You can do the following two kinds of operations any number of times in any order:

Choose two stones (not necessarily adjacent) and swap them.

Choose one stone and change its color (from red to white and vice versa).

According to a fortune-teller, a white stone placed to the immediate left of a red stone will bring a disaster. At least how many operations are needed to reach a situation without such a white stone?

Constraints

2 \leq N \leq 200000

c_i is R or W.

Input

Input is given from Standard Input in the following format:

N
c_{1}c_{2}...c_{N}

Output

Print an integer representing the minimum number of operations needed.

Sample Input 1

4
WWRR

Sample Output 1

2

For example, the two operations below will achieve the objective.

Swap the 1-st and 3-rd stones from the left, resulting in RWWR.

Change the color of the 4-th stone from the left, resulting in RWWW.

Sample Input 2

2
RR

Sample Output 2

0

It can be the case that no operation is needed.

Sample Input 3

8
WRWWRWRR

Sample Output 3

3

Solution in C (GCC 9.2.1) (judge verdict: Time Limit Exceeded)

#include <stdio.h>

int abs(int num){
        if(num<0)return num*-1;
        else return num;
}

int min(int a,int b){
        if(a<b)return a;
        else return b;
}

int countof(char c[],int from,int to,int target){
        int ans=0;
        for(int i=from;i<to;i++){
                if(c[i]==target)ans++;
        }
        return ans;
}

int main(){
        int N,ans;
        scanf("%d\n",&N);
        ans=N;
        char c[N];
        for(int i=0;i<N;i++)scanf("%c",&c[i]);

        for(int sep=0;sep<N;sep++){
                int W,R,swapNum,changeNum;
                W=countof(c,0,sep,'W');
                R=countof(c,sep+1,N,'R');
                swapNum=min(W,R);
                changeNum=abs(W-R);
                ans=min(ans,swapNum+changeNum);
        }

        printf("%d\n",ans);
        return 0;
}