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: Wrong Answer)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

int main(void){

    int i, n, rcount=0, wcount=0, count=0, ans,a[200001];
    char c;
    scanf("%d ",&n);

  for(i=0;i<n;i++){
        scanf("%c",&c);

        if(c=='R'){
            rcount++;
            if(i>0 && (char)a[i-1]=='W'){
                count++;
            }
        }

        a[i] = (int)c;
    }

    for(i=0;i<rcount;i++){
        if((char)a[i]=='W'){
            wcount++;
        }
    }

    if(wcount>count){
        ans = wcount;
    }else{
        ans = count;
    }

    printf("%d",ans);

    return 0;
}