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: Output Limit Exceeded)

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

void swap (char *x, char *y) {
    char temp;    // 値を一時保存する変数
    temp = *x;
    *x = *y;
    *y = temp;
}

int main() {
    int n, ans = 0;
    int piv1 = 0, piv2 = 0;
    int i, j, k;
    char c[200020] = {0};
    //int b[200020] = {0};//1なら消えている
    scanf("%d", &n);
    scanf("%s", c);
    i = -1;
    j = n ;

    do {
        do { i++; } while (c[i] == 'R');
        do { j--; } while (c[j] == 'W');
        if (i < j) {
            swap(&c[i],&c[j]);
            printf("%s\n",c);
            ans++;
        }
    } while(i<j);

    printf("%d",ans);
}