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 <bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define sz(a) (a).size()
#define all(c) (c).begin(), (c).end()
#define rep(i, n) for (int i = 0; i < n; i++)
#define forn(i, n) for (int i = 1; i <= n; i++)
#define ford(i, n) for (int i = n; i >= 0; i--)
#define REP(i, a, n) for (int i = a; i < n; i++)
#define foreach(itr, c) for (__typeof((c).begin(), (c).end()) itr = c.begin(); itr != c.end(); itr++)
#define fill(a, b) memset(a, b, sizeof(a))
#define re return
#define INF 1e9
#define LINF 1e18
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pi;
typedef vector<int> vi;
typedef vector<char> vs;
typedef vector<pi> vpi;

using namespace std;

int main()
{
    int N;
    int res = -1;
    int t = 0;
    int countRed = 0;
    int moves = 0;

    cin >> N;
    vs c(N);
    bool allRed = true;

    rep(i, N)
    {
        cin >> c[i];

        if (c[i] == 'W')
        {
            allRed = false;
        }
        else
        {
            countRed++;
        }
    }
    if (allRed)
    {
        cout << 0;
        return 0;
    }
    rep(i, countRed)
    {
        if (c[i] == 'W')
        {
            moves++;
        }
    }
    cout << moves;
    return 0;
}