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: Runtime Error)

#include<stdio.h>
typedef long long ll;
ll count = 0;
ll redcount = 0;
ll whitecount = 0;
ll n;
ll redposition=0;
ll whiteposition=1;
char c[2000];
ll White(ll x){
    ll i;
    for(i=x;i<=redcount;i++){
        if(c[i] == 'W'){
            return i;
        }
    }
    if(i == n-whitecount+1){
        return -1;
    }
}
ll Red(ll x){
    ll i;
    for(i=x;i<=n;i++){
        if(c[i] == 'R'){
            return i;
        }
    }
    if(i == n+1){
        return -1;
    }
}
ll swap(ll re,ll wh){
    char swap;
    swap = c[re];
    c[re] = c[wh];
    c[wh] = swap;
    count++;
}
int main(){
    ll i;
    scanf("%lld",&n);
    //printf("%lld\n",n);
    for(i=0;i<=n;i++){
        scanf("%c",&c[i]);
        if(c[i] == 'R'){
            redcount++;
        }
    }
    
    /*for(i=1;i<=n;i++){
        printf("%c\n",c[i]);
    }*/
    //printf("%lld\n",redcount);
    /*printf("%lld\n",redposition);
    printf("%lld\n",whiteposition);*/
    whitecount = n-redcount;
    redposition = redcount+1;
    //whiteposition = White(whiteposition);
    /*printf("%lld\n",redposition);
    printf("%d\n",whiteposition);*/
    if(redposition>n){
        printf("%lld\n",count);
        return 0;
    }
    while(1){
        /*Red;
        White;*/
        //printf("%lld %lld\n",redposition,whiteposition);
        if(redposition == -1){
            break;
        }else{
            swap(redposition,whiteposition);
        }
        redposition = Red(redposition);
        whiteposition = White(whiteposition);
    }
    printf("%lld\n",count);
    return 0;
}