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 fi first
#define se second
#define m_p make_pair
#define p_b push_back
#define e_b emplace_back
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
using namespace std;
using ll = long long;
using ld = long double;
template<class T> bool chmax(T &a,T b){if(a<b){a=b;return true;}return false;}
template<class T> bool chmin(T &a,T b){if(a>b){a=b;return true;}return false;}

int main(){
  ios_base::sync_with_stdio(false);cin.tie(0);
  int N;
  string S;
  cin >> N >> S;

  int cnt=0;
  for(int i=0;i<N;i++)  if(S.at(i)=='R')  cnt++;
  int ans=0;
  for(int i=0;i<cnt;i++)  if(S.at(i)=='W')  ans++;
  cout << ans << endl;

  return 0;
}