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>
#include <string.h>
#define ll long long
#define fr(i,n,m) for(ll i=n;i<m;i++)
int main()
{
    ll n,rcount=0, ans;
    scanf("%lli",&n);
    char c[200005];
    scanf("%s",c);
    fr(i,0,n)
    {
        if (c[i] == 'R') rcount++;
    }
    ll left=0;
    fr(i,0,rcount)
    {
        if (c[i] == 'W') {
            left++;
        }
    }
    ll right=0;
    fr (i,0,n-rcount)
    {
        if (c[n - i - 1] == 'R') {
            right++;
        }
    }
    if (left > right) {
        ans = left;
    } else {
        ans = right;
    }
    printf("%lli\n",ans);
    return 0;
}