#include <bits/stdc++.h>
using namespace std;
#define REP(i,x,n) for(int i=x;i<(int)n;i++)
#define rep(i,n) REP(i,0,n)
#define sp(p) cout<<setprecision(16)<<fixed<<p<<endl;
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define SORT(a) sort(all(a))
#define RSORT(a) sort(rall(a))
#define UNIQ(a) unique(all(a))
#define UNIQNUM(a) UNIQ(a)-a.begin()
#define UNIQIT(a) a.erase(UNIQ(a), a.end());
#define VOUT(v,i) rep(i,v.size())cout<<v[i]<<(i==v.size()-1?"\n":" ");
#define vout(v) VOUT(v,z);
#define vdbg(v,i) cout<<#v<<": ";for(int i=0;i<(int)v.size();i++){cout<<v[i]<<" ";}cout<<"\n";
#define vmin(v) *min_element(all(v))
#define vmax(v) *max_element(all(v))
#define vsum(v) accumulate(all(v), 0LL)
#define MOUT(m,r,c) rep(i,r){rep(j,c){cout<<m[i][j]<<" ";}cout<<endl;}
#define mout(m) MOUT(m,m.size(),m[0].size())
#define debg(a) cout<<#a<<": "<<a<<endl;
#define show(a) for(cont &y:a){for(cont &x:y){cout<<x<<" ";}cout<<endl;}
#define digit(a) to_string(a).length();
template<class T>inline int out(const T &t){ print(t); putchar('\n'); return 0; }
// template<class T>inline T gcd(T a,T b){if(b==0)return a; return(gcd(b,a%b));}
// template<class T>inline T lcm(T a,T b){return a/gcd(a,b)*b;}
bool is_palindrome(string s){return s == string(s.rbegin(),s.rend());}
#define popcount __builtin_popcount
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> P;
typedef vector<ll> V;
typedef vector<vector<ll>> VV;
// const long long MOD=1000000007;
const long long INF = 1e18;
#define EPS (1e-7)
#define PI (acos(-1))
template<class T>
inline bool chmax(T &a, T b) {
    if(a < b) { a = b; return true; }
    return false;
}
template<class T>
inline bool chmin(T &a, T b) {
    if(a > b) { a = b; return true; }
    return false;
}



int main(){
    ll N;
    cin >> N;
    vector<char> c(N);
    for (long long i = 0; i < N; i++) {
        cin >> c[i];
    }
    // vout(c)
    ll rcnt = count(all(c), 'R');
    ll wcnt = count(all(c), 'W');
    if(rcnt==N){
        cout << 0 << endl;
        return 0;
    }else if(wcnt==N){
        cout << 0 << endl;
        return 0;
    }

    // ll rpos=N;
    // ll lwcnt=0;
    // for (long long i = 0; i < N; i++) {
    //     if(c[i]=='W') {
    //         lwcnt++;
    //         continue;
    //     }
    //     if(c[i]=='R') {
    //         rpos=i;
    //         break;
    //     }
    // }
    // for (long long i = rpos; i < N; i++) {
    //     if(c[i]=='R') {
    //         rpos=i;
    //     }else{
    //         break;
    //     }
    // }
    // ll cnt=lwcnt;
    // for (long long i = N-1; i >= 0; i--) {
    //     if(c[i]=='R' && lwcnt>0){
    //         c[i]='W';
    //         lwcnt--;
    //     }
    //     if(lwcnt==0) break;
    // }
    // ll rrcnt=0;
    // for (long long i = rpos+1; i < N; i++) {
    //     if(c[i]=='R') rrcnt++;
    // }
    // cnt+=rrcnt;

    ll cnt=0;
    ll j=N-1;
    for (long long i = 0; i < N; i++) {
        if(c[i]=='W' && i<j){
            c[i]='R';
            while(c[j]=='W') j--;
            if(i>=j) break;
            c[j]='W';
            cnt++;
        }
        if(i>=j) break;
        // vout(c);
        // debg(cnt);
    }

    cout << min({rcnt, wcnt, cnt}) << endl;

    return 0;
}


Measured memory_bytes: 3604