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<iostream>
#include<cmath>
#define END "\n"
#define REP(i, a, b) for (int i = a; i < b; ++i)

using namespace std;  
typedef long long ll;

int main () {
    ios::sync_with_stdio(0);
    cin.tie(0);
    int n, count = 0;
    char c, p;
    int first = 0;
    cin >> n;
    cin >> p;
    if (p == 'R') first = 1;
    REP(i, 1, n) {
        cin >> c;
        if ((first == 0) && (c =='R')) first = i+1;
        if ((c == 'R') && (p == 'W')) {
            count++;
            p = 'W';
        } else p = c;
    }
    if ((count - 1) >= first) count--;
    cout << count;
    return 0;
}