#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
const ll INF = 1e9;
const ll MOD = 1e9 + 7;
#define all(v) v.begin(), v.end()
#define rep(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
#define repi(i, n, init) for(ll i=init;i<(n);i++)
#define repd(i, n, init) for(ll i=(n);i>=init;i--)
#define repm(i, m) for(auto i=m.begin();i!=m.end();i++)
#define repb(bit, n, init) for(ll bit=init;bit<(1<<n);++bit)

//debug の define
#define DEBUG_

// vector
template<typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}

// pair
template<typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
    os << "(" << pair_var.first << ", " << pair_var.second << ")";
    return os;
}

// vector
template<typename T>
ostream &operator<<(ostream &os, const vector<T> &vec) {
    os << "{";
    for (int i = 0; i < vec.size(); i++) {
        os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
}

// map
template<typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
    os << "{";
    rep(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}

// set
template<typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
    os << "{";
    rep(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) os << ", ";
        itr--;
    }
    os << "}";
    return os;
}

#define DUMPOUT cerr

void dump_func() {
    DUMPOUT << endl;
}

template<class Head, class... Tail>
void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) {
        DUMPOUT << ", ";
    }
    dump_func(std::move(tail)...);
}

#ifdef DEBUG_
#define DEB
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif

vector<ll> fact, invfact;

ll modpower(ll x, ll n, ll mod) {
    ll res = 1;
    while (n) {
        if (n & 1)res = res * x % mod;
        x = x * x % mod;
        n >>= 1;
    }
    return res;
}

ll inv(ll n) {
    return modpower(n, MOD - 2, MOD);
}

void COMinit(ll k) { //空っぽの配列を二つ
    k++;
    fact.resize(k, 0);
    invfact.resize(k, 0);
    fact[0] = 1;
    repi(i, k, 1) {
        fact[i] = fact[i - 1] * i % MOD;
    }
    invfact[k - 1] = inv(fact[k - 1]);
    repd(i, k - 2, 0) {
        invfact[i] = invfact[i + 1] * (i + 1) % MOD;
    }
    return;
}

ll nCk(ll n, ll k) { //この関数を使う前にCOMinit
    if (k < 0 || n < k)
        return 0LL;
    return fact[n] * invfact[k] % MOD * invfact[n - k] % MOD;
}

ll com(ll n, ll k) {
    ll iv = 1;
    ll fa = 1;
    repi(i, k, 0) {
        fa *= (n - i);
        fa %= MOD;
    }
    repi(i, k + 1, 1) {
        iv *= inv(i);
        iv %= MOD;
    }
    return fa * iv % MOD;
}

//---------main function----------

int main() {
    int n;
    cin >> n;
    string c;
    cin >> c;
    int r = 0;
    repi(i, n, 0) {
        if (c[i] == 'R')r++;
    }
    int ans = 0;
    repi(i, r, 0) {
        if (c[i] == 'W')ans++;
    }
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 3692