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>

using namespace std;
using ll = long long;
using vl = vector<ll>;
using pll = pair<ll, ll>;

#define rep(i, n) for (ll i = 0, xxx_rep_end = (n); i < xxx_rep_end; ++i)
#define all(x) (x).begin(), (x).end()

template <class T1, class T2>
inline void chmax(T1 &a, const T2 &b)
{
    if (a < b)
    {
        a = b;
    }
}
template <class T1, class T2>
inline void chmin(T1 &a, const T2 &b)
{
    if (a > b)
    {
        a = b;
    }
}

constexpr ll INF = 0x3f3f3f3f3f3f3f3fLL;

int main()
{
    ll N;
    cin >> N;
    string c;
    cin >> c;
    ll w_cnt = 0;
    ll r_cnt = 0;
    rep(i, c.size())
    {
        if (c[i] == 'W')
        {
            ++w_cnt;
        }
        else
        {
            ++r_cnt;
        }
    }

    ll ans = 0;
    rep(i, r_cnt)
    {
        if (c[i] == 'W')
        {
            ++ans;
        }
    }
    cout << ans << endl;
}