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: Accepted)

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

int main(int argc, char const *argv[])
{
    unsigned long N;
    scanf("%lu", &N);

    char *c;
    c = malloc(sizeof(char) * (N + 1));

    unsigned cr = 0, cw = 0;
    scanf("%s", c);

    for (size_t i = 0; i < N; i++)
    {
        if (c[i] == 'R')
        {
            cr++;
        }
        else
        {
            cw++;
        }
    }

    unsigned count = 0;
    for (size_t i = 0; i < cr; i++)
    {
        if (c[i] == 'W')
        {
            count++;
        }
    }

    printf("%u\n", count);

    return 0;
}