#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds; 
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define ll long long int
#define fio ios_base::sync_with_stdio(false); cin.tie(NULL);
#define MAX_LIMIT 10000000
#define all(x) x.begin(),x.end()
#define rs resize
#define remove_zero(str) str.erase(0, str.find_first_not_of('0'));
#define br break
const long long kot = LLONG_MAX;
#define pb push_back
#define pi acos(-1)
#define pp pop
#define pf pop_front
using ld = long double;
#define endl "\n"
#define rese(x,y) memset(x,y,sizeof(x))
#define ff first
#define ss second
#define forn(i, n) for (ll i = 0; i < ll(n); i++)
#define np next_permutation
const int MAXIT = 100; 
#define ul unsigned long
#define pii pair<ll,ll>
#define cc continue
#define ee end
#define ii insert
#define fr front
#define mod 1000000007
#define mod1 998244353
#define pbb pop_back
#define dd distance
#define up upper_bound
#define pc pop_back()
#define lb lower_bound
#define pu push
#define to top
//#define r reverse
#define bb back
#define CHARS_TOTAL 256
#define INT_BITS 32 
#define SIZE 26
//#pragma GCC optimize "trapv"
//#define _GLIBCXX_DEBUG
//typedef std::pair<std::string,std::string>pair;
const ll alpha=1e18;
/* For counting number of digits, directly do floor(log10(n)+1)*/
using namespace std; 
ll gcd(ll a, ll b)
{
    // Everything divides 0
    if (b == 0) 
        return a; 
    return gcd(b, a % b); 
}
ll powe(ll x, unsigned ll y) 
{ 
    ll res = 1;      // Initialize result 
  
     // Update x if it is more than or  
                // equal to p 
  
    while (y > 0) 
    { 
        // If y is odd, multiply x with result 
        if (y & 1) 
            res = (res*x); 
  
        // y must be even now 
        y = y>>1; // y = y/2 
        x = (x*x);   
    } 
    return res; 
} 
// Function to return LCM of two numbers
 ll lcm(ll a, ll b)
 {
    return (a*b)/gcd(a, b);
 } 
bool isPerfectSquare(long double x)
{
  long double sr = sqrt(x);
  return ((sr - floor(sr)) == 0);
}
bool comp(ll a, ll b)
{
    return (a < b);
}
vector<ll>divisors(ll n)
{
    vector<ll>ve;
    for (ll i=1; i<=sqrt(n); i++)
    {
        if (n%i == 0)
        {
            // If divisors are equal, print only one
            if (n/i == i)
                ve.pb(i);
 
            else // Otherwise print both
            ve.pb(i);
            ve.pb(n/i);
        }
    }
    return ve;
}
  
bool isPrime(ll n) {
 
    if (n == 1) {
        return false;
    }
 
    ll i = 2;
 
    while (i*i <= n) {
 
        if (n % i == 0) {
             return false;
        }
        i += 1;
    }
    return true;
}
ll gcdofarray(vector<ll>&v, ll n)
{
    ll result = v[0];
    for (ll i = 1; i < n; i++)
        result = gcd(v[i], result);
    return result;
}
 
ll sumofdigits(ll n){
        ll sum=0;
        while(n>0){
            sum+=n%10;
            n/=10;
        }
        return sum;
}
bool powertwo(ll x){
  return x&& (!(x&(x-1)));
}
string removeDuplicatesFromString(string str)
{
    ll counter = 0;
 
    ll i = 0;
    ll size = str.size();
    ll x;
 
    // keeps track of length of resultant string
    ll length = 0;
 
    while (i < size) {
        x = str[i] - 97;
 
        // check if Xth bit of counter is unset
        if ((counter & (1 << x)) == 0) {
 
            str[length] = 'a' + x;
 
            // mark current character as visited
            counter = counter | (1 << x);
 
            length++;
        }
        i++;
    }
 
    return str.substr(0, length);
}
ll commDiv(ll a,ll b)
{
    // find gcd of a,b
    ll n = gcd(a, b);
 
    // Count divisors of n.
    ll result = 0;
    for (ll i=1; i<=sqrt(n); i++)
    {
        // if 'i' is factor of n
        if (n%i==0)
        {
            // check if divisors are equal
            if (n/i == i)
                result += 1;
            else
                result += 2;
        }
    }
    return result;
}
ll kash(ll a,ll b){
    return (a*b)/gcd(a,b);
}
ll maxi(ll a,ll b){
   return (a>b)?a:b;
}
ll mini(ll a,ll b){
   return (a<b)?a:b;
}
bool sortByVal(const pair<ll,ll> &a, const pair<ll,ll> &b){ 
    
    return a.second>b.second;
} 

//vector<ll>arr[1001];
//ll vis[1001];
//pair<ll,ll>p[1001];
/*void dfs(ll node){
    if(vis[node])return;
    vis[node]=1;
    for(ll i=0;i<n;i++){
        if (p[i].first == p[node].first || p[node].second == p[i].second){
            dfs(i);
        }
    }
}*/
/*ll check(ll arr[],ll n,ll v){
    ll dp[v+1];
    dp[0]=0;
    ll i,j;
    for(i=1;i<=v;i++){
        dp[i]=INT_MAX;
    }
    for(i=1;i<=v;i++){
        for(j=0;j<n;j++){
            if(arr[i]<=i){
                ll y=dp[i-arr[j]];
                if(y!=INT_MAX && y+1<arr[i]){
                    dp[i]=y+1;
                }
            }
        }
    }
    return dp[v];
}
*/
bool isvowel(char ch){
    if(ch=='a' || ch=='e' || ch=='i'|| ch=='o'|| ch=='u'){
        return true;
    }
    return false;    
}
ll power(ll x,ll y, ll p)  
{  
    ll res = 1;     // Initialize result  
  
   // x = x % p; // Update x if it is more than or  
                // equal to p  
  
    while (y > 0)  
    {  
        // If y is odd, multiply x with result  
        if (y & 1)  
            res = (res*x) % p;  
  
        // y must be even now  
        y = y>>1; // y = y/2  
        x = (x*x) % p;  
    }  
    return res;  
} 
ll parent[100001];
ll find(ll a){
    while(parent[a]>0){
        a=parent[a];
    }    
    return a;
}
void Union(ll a,ll b){
     parent[a]+=parent[b];
     parent[b]=a;    
}
bool ispalind(string str){
     string s=str;
     //reverse(str.begin(),str.end());
     if(s==str)return true;
     return false;    
}
//ll fact[N], invfact[N];

ll modinv(ll k)
{
    return power(k, mod1-2, mod1);
}

/*void precompute()
{
    fact[0]=fact[1]=1;
    for(ll i=2;i<N;i++)
    {
        fact[i]=fact[i-1]*i;
        fact[i]%=mod;
    }
    invfact[N-1]=modinv(fact[N-1]);
    for(ll i=N-2;i>=0;i--)
    {
        invfact[i]=invfact[i+1]*(i+1);
        invfact[i]%=mod;
    }
}

ll nCr(ll x, ll y)
{
    if(y>x)
        return 0;
    ll num=fact[x];
    num*=invfact[y];
    num%=mod;
    num*=invfact[x-y];
    num%=mod;
    return num;
}
/*
ll arr[3];
ll I[3][3],T[3][3];
void mul(ll A[3][3],ll B[3][3],ll dim){
        ll res[dim+1][dim+1];
        ll i,j,k;
        for(i=1;i<=dim;i++){
            for(j=1;j<=dim;j++){
                res[i][j]=0;
                for(k=1;k<=dim;k++){
                    //res[i][j]+=A[i][j]*B[k][j];
                    ll x=(A[i][k]*B[k][j])%mod;
                    res[i][j]=(res[i][j]+x)%mod;
                }
            }
        }    
         for(i=1;i<=dim;i++){
            for(j=1;j<=dim;j++){
                A[i][j]=res[i][j];
            }
        }    
}
*/
/*
ll getfib(ll n){
     if(n<=2) return arr[n];
     I[1][1]=I[2][2]=1;
     I[1][2]=I[2][1]=0;
     
     T[1][1]=0;
     T[1][2]=-1;
     T[2][1]=1;
     T[2][2]=3;
     n=n-1;
     while(n){
        if(n%2==1){
            mul(I,T,2);
            n--; 
        }
        else{
            mul(T,T,2);
            n=n/2;
        }
     }
     ll x=(arr[1]*I[1][1]+arr[2]*I[2][1])%mod;
     return x;
}*/
/*vector<ll>graph[103];
bool vis[103];
void dfs(ll u){
    if(vis[u]) return;
    vis[u]=true;
    for(auto it:graph[u]){
        dfs(it);
    }
}*/ 
//3->8
//s->

//2 3 1
//2 2 2
//1 
bool palin(string s){
    string s1=s;
    reverse(all(s));
    if(s1==s){
        return true;
    }
    return false;
}
//a
//aa
//aaaa
//aaaa
//aa

// To use lambda function- 
/*function<ll(ll,ll)>sum=[&](ll x,ll y){
         return x+y;
     };
*/
  
void solve(){
     //input
    ll t, i, x, j, y, 	z, k, n;
	string s;
	cin >> n >> s;
	z = 0;
	while (s.size() and s.back() == 'W')	s.pop_back();
	n = s.size();

	j = n - 1;

	std::vector<pair<char, ll>> v;

	for(ll i=0;i<n;i++){
		if (v.size() and v.back().ff == s[i])	v.back().ss++;
		else v.push_back({s[i], 1});
	}
	j = v.size() - 1;
	for(ll i=0;i<v.size();i++) {
		if (v[i].ff == 'W' and i + 1 < v.size() and v[i + 1].ff == 'R'  and v[i + 1].ss) {
			while (1) {
				while (j > i and v[j].ff != 'R') {
					j--;
				}
				if (i == 2) {
					// debug(j);
				}
				if (j > i and v[j].ff == 'R') {
					// if (v[j].ss == 0)	j--;
					// else {
					if (v[j].ss > v[i].ss) {
						z += v[i].second;v[j].second -= v[i].second;
						v[i].second = 0;
						break;
					} else {z += v[j].second;v[i].second -= v[j].second;v[j].second = 0;j--;
					}
				}

				else {
					if (v[i + 1].second < v[i].second) {
						z += v[i + 1].second;
					}
					else
						z += v[i].second;
					break;
				}
			}
		}
	}
	cout<<z<<endl;

     
     
     
}
//3 3
//4 1 2 3->1 2 4
//5 1 2 5->8
//2 8    
int32_t main()
{    
     fio;
    //input
   // bruteforce();
   ll t=1;
   //cin>>t;
   while(t--){
         solve();
   }
}


Measured memory_bytes: 5524