#pragma GCC optimize("O3")
#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
using namespace std;
using base = complex<double>;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define M 1000000007
#define M2 998244353
#define ll long long
#define pll pair<long, long>
#define REP(i, a, b) for (ll i = a; i < b; i++)
#define REPI(i, a, b) for (ll i = b - 1; i >= a; i--)
#define ff first
#define ss second
#define pb push_back
#define db pop_back
#define mp make_pair
#define mt make_tuple
#define g(a, b) get<a>(b)
#define INF (ll)1e18 + 100
#define vl vector<ll>
#define vi vector<int>
#define vll vector<pair<ll, ll>>
#define vii vector<pair<int, int>>
#define all(v) v.begin(), v.end()
#define bset(a, p) ((a) | (1ll << (p)))
#define bchk(a, p) ((a) & (1ll << (p)))
#define bxor(a, p) ((a) ^ (1ll << (p)));
#define brem(a, p) (bchk(a, p) ? (bxor(a, p)) : (a))
#define INT_SIZE 32
/*SOME BITMASK KNOWLEDGE
1)x & (x - 1):sets the last one bit of x to zero
power of two exactly when x & (x − 1) = 0.
2)x & -x:sets all the one bits to zero, except last one bit
3)x | (x - 1):inverts all the bits after the last one bit*/

#define o_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define o_setpll tree<pair<ll,ll>, null_type, less<pair<ll,ll>>, rb_tree_tag, tree_order_statistics_node_update>
//member functions :
//1. order_of_key(k) : number of elements strictly lesser than k
//2. find_by_order(k) : k-th element in the set
ll powM(ll x, ll y, ll m)
{
    ll ans = 1, r = 1;
    x %= m;
    while (r > 0 && r <= y)
    {
        if (r & y)
        {
            ans *= x;
            ans %= m;
        }
        r <<= 1;
        x *= x;
        x %= m;
    }
    return ans;
}
ll modI(ll a, ll m)
{
    ll m0 = m, y = 0, x = 1;
    if (m == 1)
        return 0;
    while (a > 1)
    {
        ll q = a / m;
        ll t = m;
        m = a % m;
        a = t;
        t = y;
        y = x - q * y;
        x = t;
    }
    if (x < 0)
        x += m0;
    return x;
}
ll logint(ll x, ll y)
{
    ll ans = 0;
    ll a = 1;
    for (ll i = 0; i <= x; i++)
    {
        if (x < a)
        {
            return ans;
        }
        ans++;
        a *= y;
    }
    return -1;
}
ll extended(ll a, ll b, ll &x, ll &y)
{
    if (a == 0)
    {
        x = 0;
        y = 1;
        return b;
    }
    ll x1, y1;
    ll d = extended(b % a, a, x1, y1);
    x = y1 - (b / a) * x1;
    y = x1;
    return d;
}
ll lcm(ll x, ll y) {
    ll gc = __gcd(x, y);
    x /= gc;
    if ((ll)(8e18) / y <= x) {
        return 8e18;
    }
    return (x * y);
}
ll power(ll x, ll y) {
    ll curr = 1;
    REP(i, 0, y) {
        curr *= x;
    }
    return curr;
}
struct matrix
{
    vector <vector <ll> >m;
};
void msi(matrix &m, ll a, ll b)
{
    m.m.resize(a);
    REP(i, 0, a) {
        m.m[i].resize(b);
    }
}
void minit(matrix &m, ll x)
{
    REP(i, 0, m.m.size())
    REP(j, 0, m.m[0].size())
    {m.m[i][j] = x;}
}
matrix mul(matrix m1, matrix m2)
{
    matrix m3;
    msi(m3, m1.m.size(), m2.m[0].size());
    REP(i, 0, m3.m.size())
    {
        REP(j, 0, m3.m[0].size())
        {   m3.m[i][j] = 0;
            REP(k, 0, m1.m[0].size())
            m3.m[i][j] = (m3.m[i][j] + m1.m[i][k] * m2.m[k][j]) % M;
        }
    }
    return m3;
}
matrix mexp(matrix &mx, ll p)
{
    matrix res;
    msi(res, mx.m.size(), mx.m.size());
    minit(res, 0);
    REP(i, 0, mx.m.size())
    res.m[i][i] = 1;
    while (p > 0)
    {
        if (p & 1)
            res = mul(res, mx);
        mx = mul(mx, mx);
        p = p >> 1;
    }
    return res;
}
const int N = 1e5 + 5;

void solve() {
    // ll x;
    // cin >> x;
    // if (x >= 30) {
    //     cout << "Yes";
    // }
    // else {
    //     cout << "No";
    // }
    ll n;
    cin >> n;
    string s;
    cin >> s;
    ll red = 0, white = 0;
    REP(i, 0, n) {
        if (s[i] == 'W') {white++;}
        else {
            red++;
        }
    }
    if (red == 0 || white == 0) {
        cout << 0; return;
    }
    ll r = red;
    REP(i, 0, red) {
        if (s[i] == 'R') {
            r--;
        }
    }
    cout << r;

}
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    ll ntc;
    // cin >> ntc;
    ntc = 1;
    REP(tc, 1, ntc + 1)
    {
        // cout << "Case #" << tc << ": ";
        solve();
    }
}

Measured memory_bytes: 3724