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;

const int maxN = 2e5+5;

void solve(){
    int n; cin >> n;
    int A[maxN];
    int count_red = 0, count_red2 = 0;
    for(int i = 0; i < n; i++){
        char c; cin >> c;
        if(c == 'W') A[i] = 0;
        else{
            A[i] = 1;
            count_red++;
        }
    }
    for(int i = count_red; i < n; i++){
        if(A[i] == 1) count_red2++;
    }
    int ans = 0;
    int white = n-count_red+count_red2;
    ans = white;
    if(white < count_red2){
        count_red2 -= white;
        white = 0;
    }
    else{
        white -= count_red2;
        count_red2 = 0;
    }

    cout << ans+count_red2 << "\n";


}


int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);

    bool multitest = false;
    //multitest = true;
    if(multitest){
        int t; cin >> t;
        while(t--)
            solve();
    }
    else
        solve();
}