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>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef long long ll;

const int mod=1e9+7;
const int inf=-1;
const int sz=2e5;

#define pi acos(-1);
#define pb push_back;
#define mp make_pair;
#define ff first;
#define ss second;
#define gap ' ';
#define lb lower_bound;
#define ub upper_bound;
#define clr(x) memset(x,0,sizeof(x));
#define all(x) x.begin(),x.end();
#define rall(x) x.begin(),x.end();
#define dbg puts("finding wrong");
#define case(x,y) printf("Case %lld: %lld\n",++x,y);
#define fastio ios::sync_with_stdio(false);cin.tie(0);

typedef vector<ll> v;
typedef map<ll,ll>use;
typedef set<ll>st;
typedef queue<ll>q;
typedef deque<ll>dq;
typedef pair<ll,ll>pa;
typedef vector<pa>vii;

int ar[sz];

int main()
{
  fastio;
  ll r=0, w=0, ans=0, i, n, j;
  string s;

  cin>>n>>s;

  for(i=0, j=n-1; i<n and j>=0; i++, j--)
  {
   while(s[i]=='R' and i<n) i++;

   while(s[j]=='W' and j>=0) j--;

   if(i<j and i<n and j>=0)
   {
   	ans++;	
   	swap(s[i],s[j]);
   }
  }

  printf("%lld\n",ans);
}