#include <bits/stdc++.h>
//#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
//using namespace __gnu_pbds;
  #define ll  long long
    #define rep(i,n) for(ll i=0;i<n;i++)
    #define pb push_back
    #define in insert
    #define pll pair<ll,ll>
    #define mll map<ll,ll>
    #define si set<ll>
    #define sc set<char>
    #define msi multiset<ll>
    #define msc multiset<char>
//typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;

    #define test   ll t;cin>>t;while(t--)
   // #define bs  binary_search
    #define rep1(x,n) for(ll x=n-1;x>=0;x--) 
    #define for1(i,a,n) for(ll i=a;i<n;i++)
    #define for2(i,a,n) for(ll i=n-1;i>=a;i--)
    #define F1 first
    #define S2 second
    #define vi vector<ll>
    #define vii vector<vector<ll>>
    #define vpi vector<pll>
    #define SORT(x) sort(x.begin(),x.end())
    #define ERASE(x) x.erase(unique(x.begin(),x.end()),x.end())
    #define All(x) (x).begin(),(x).end()
    #define rAll(x) (x).rbegin(),(x).rend()
    #define db  double
    #define S size()
    #define EN end()
    #define BE begin()
    #define LEN length()
    #define itr(s) for(auto x:s)


    
    const ll MAX=1e9+1;
    
    const double pi=3.1415926535897932384626433832;
    const ll M=2e18 +7;
    ll bis(ll a[],ll l,ll r,ll x){
     while(l<=r){
      ll m=l +(r-l)/2;
      if(a[m]==x)
      return m;
      if(a[m]<x)l=m+1;
      else r=m-1;
     }
     return -1;
    }
    
    ll gcd(ll x,ll y){
    	if(x==0)return y;
    	else return gcd(y%x,x);
    }
    ll lcm(ll x,ll y){
    	return (x*y)/gcd(x,y);
    }
   
bool isPrime(ll n) 
{  
    if (n <= 1) 
        return false; 
    if (n <= 3) 
        return true; 
    if (n % 2 == 0 || n % 3 == 0) 
        return false; 
  
    for (ll i = 5; i * i <= n; i = i + 6){ 
        if (n % i == 0 || n % (i + 2) == 0){ 
            return false; 
        }
    }
    return true; 
} 
bool pdrome(string s){
    ll n = s.length();
    for(ll i=0;i<n/2;i++){
        if(s[i]!=s[n-i-1]){
            return false;
        }
    }
    return true;
}
ll mindig(ll x){
    ll ans = 9;
    ll t = x;
    while(x){
        ans = min(ans,x%10);
        x/=10;
    }
    x = t;
    return ans;
}
ll maxdig(ll x){
    ll ans = 0;
    ll t = x;
    while(x){
        ans = max(ans,x%10);
        x/=10;
    }
    x = t;
    return ans;
}
ll modpow(ll x,ll n,ll M){
	if(n==0)return 1%M;
	ll u=modpow(x,n/2,M);
	u=(u*u)%M;
	if(n%2==1)u=(u*x)%M;
	return u;

}
ll sum(ll a) {
    ll result = 0;
    while (a > 0) {
        result += a % 10;
        a /= 10;
    }
    return result;
}
ll digits(long long n) 
{ 
    return floor(log10(n) + 1); 
}
const ll Node=1e5+7;
vi adj[Node];
bool vis[Node];
void addedge(ll u,ll v){
    adj[u].pb(v);
    adj[v].pb(u);
}
void bfs(ll u){
    vis[u]=true;
    list<ll>q;
    q.pb(u);
    while(!q.empty()){
        //cout<<q.front()<<" ";
        q.pop_front();
        for(auto x:adj[u]){
            if(!vis[x]){
                vis[x]=true;
                q.pb(x);
            }
        }
    }
}
void dfs(ll u){
    vis[u]=true;
    for(auto x:adj[u]){
        if(!vis[x]){
        dfs(x);
        }
    }
}
ll con1(string s){
    long long res=0;
    stringstream geek(s); 
    geek>>res;
    return res;
    //string stri = boost::lexical_cast<string>(i_val);  
}
string con2(ll num){
    //string stri = boost::lexical_cast<string>(num);
    string stri=to_string(num);
    return stri;   
}
/* int num_to_bits[16] = { 0, 1, 1, 2, 1, 2, 2, 3, 
                        1, 2, 2, 3, 2, 3, 3, 4 }; 
  
 Recursively get nibble of a given number  
and map them in the array 
unsigned int cntbit(unsigned int num) 
{ 
    ll nibble = 0; 
    if (0 == num) 
        return num_to_bits[0]; 
  
    // Find last nibble 
    nibble = num & 0xf; 
  
    // Use pre-stored values to find count 
    // in last nibble plus recursively add 
    // remaining nibbles. 
    return num_to_bits[nibble] + cntbit(num >> 4); 
} 
  */

//string stri = boost::lexical_cast<string>(i_val); 
const ll N=2e5+7;
//ll dp[N]; 

void solve(){
ll n,nr=0;
cin>>n;
string s;
cin>>s;
rep(i,n){
    if(s[i]=='R')nr++;
} string res(nr,'R');
for1(i,0,n-nr){
    res+='W';
}ll ans=0,k=min(nr,n-nr);
for1(i,0,n){
    if(res[i]!=s[i])ans++;
}
cout<<min(ans/2,k);


 
return;
}
int main() {
    	
        #ifndef ONLINE_JUDGE
               freopen("input.txt","r",stdin);
               freopen("output.txt","w",stdout);
               #endif
               ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
       
    solve();
        return 0;
}
/*Yes
Yes
No
Yes
Yes
No
Yes
Yes
*/

Measured memory_bytes: 6200