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>
 
#pragma GCC optimize("O2")
#pragma GCC optimize("unroll-loops")
 
using namespace std;
#define ll long long
#define ull unsigned long long
#define rep(i,n,N) for(int i=n;i<=N;++i)
#define asd(i,n,N) for(int i=n;i< N;++i)
#define rap(i,n,N) for(int i=n;i>=N;--i)
#define mp make_pair
#define pb push_back
#define pob pop_back
#define pf push_front
#define pof pop_front
#define fi first
#define se second
#define ff fi.fi
#define fs fi.se
#define sf se.fi
#define ss se.se
#define lc (id<<1)
#define rc ((id<<1)|1)
#define db(x) cout << ">>>> " << #x << " -> " << x << endl
#define mems(x,y) memset(x,y,sizeof x)
#define all(x) x.begin(),x.end()
#define pii pair<int,int> 
#define pll pair<ll,ll>
#define plll pair<pll,ll>
#define piii pair<pii,int>
#define piiii pair<pii,pii>
#define psi pair<string,int>
#define endl '\n'
const int MAX = 1e6+5;
const ll MAX2 = 11;
const ll MOD = 1000000007;
const ll MOD2 = 1000005329;
const ll INF = 2e18;
const int dr[]={1,0,-1,0,1,1,-1,-1,0};
const int dc[]={0,1,0,-1,1,-1,1,-1,0};
const double pi = acos(-1);
const double EPS = 1e-9;
const int block = 555;

ll n,ans,ri;
char x[MAX];

int main(){
//	cout<<fixed<<setprecision(10);
//	freopen("input.txt", "r", stdin);
//	freopen("astar.out","w",stdout);
    ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
    cin>>n;
    rep(i,1,n)cin>>x[i];
    ri = n;
    rep(i,1,n){
    	while(x[ri]=='W')--ri;
    	if(i>ri)break;
    	if(x[i]=='W')swap(x[i],x[ri]), ++ans;
	}
	cout<<ans<<endl;
	return 0;
}