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 <bits/stdc++.h>
using namespace std;
void print() {
    cout << endl;
}
template <class Head, class... Tail>
void print(Head &&head, Tail &&... tail) {
    cout << head;
    if (sizeof...(tail) != 0)
        cout << " ";
    print(forward<Tail>(tail)...);
}
template <class T>
void print(vector<T> &vec) {
    for (auto &a : vec) {
        cout << a;
        if (&a != &vec.back())
            cout << " ";
    }
    cout << endl;
}
template <class T>
void print(vector<vector<T>> &df) {
    for (auto &vec : df) {
        print(vec);
    }
}
#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define xrep(i, a, b) for (int i = int(a); i < int(b); ++i)
using ll = long long;
const int INF = 1001001001;

int main() {
    int N;
    cin >> N;
    string C;
    cin >> C;
    int mid = N/2;
    int cnt = 0;
    rep(i, N) {
        if(C[i]=='W') {
            xrep(j, mid, N) {
                if(C[j]=='R') {
                    swap(C[i], C[j]);
                    cnt++;
                    break;
                }
            }
        }
    }
    print(cnt);
    return 0;
}