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: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;
int main(void){
    // Your code here!
    long long N,r,w,red,white;cin>>N;r=w=red=white=0;
    vector<char>C(N);
    for(long long i=0;i<N;i++){
        cin>>C[i];
        if(C[i]=='W'){
            white++;
            if(i<N/2){
                w++;
            }
        }
        if(C[i]=='R'){
            red++;
            if(N/2<=i){
                r++;
            }
        }
    }
    if(white==0||red==0){
        cout<<0<<endl;
        return 0;
    }
    long long ans=0;
    if(w==r){
        ans+=r;
        cout<<r<<endl;
        return 0;
    }
    ans+=min(w,r);
    if(w>r){
        ans+=(w-r);
    }
    cout<<ans<<endl;
    return 0;
}