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>
int main()
{
    int n,i,j,tct=0,ct=0;
    scanf("%d\n",&n);
    char arr[n+1];
    scanf("%s",arr);
    i=0;
    j=n-1;
    while(1)
    {
        if(arr[i]=='W') 
        {
            ct++;
            if(arr[i+1]=='R')
            {
                while(ct>0)
                {
                    if(arr[j]=='R')
                    {
                        ct--;
                        tct++;
                    }
                    j--;
                    if(j==i) goto end;
                }
            }
        }
        i++;
        if(j==i) goto end;
    }
    end:
    {
        printf("%d",tct);
    }
    return 0;
}