#include "bits/stdc++.h"
#include "ext/pb_ds/assoc_container.hpp"
#include "ext/pb_ds/tree_policy.hpp"
#include "ext/rope"

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;

//#pragma GCC target ("avx2")
//#pragma GCC optimization ("O3")
//#pragma GCC optimization ("unroll-loops")
//#pragma comment(linker, "/stack:200000000")
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

void trace(int x)                         { cerr << x; }
void trace(long x)                        { cerr << x; }
void trace(long long x)                   { cerr << x; }
void trace(unsigned x)                    { cerr << x; }
void trace(unsigned long x)               { cerr << x; }
void trace(unsigned long long x)          { cerr << x; }
void trace(float x)                       { cerr << x; }
void trace(double x)                      { cerr << x; }
void trace(long double x)                 { cerr << x; }
void trace(char x)                        { cerr << '\'' << x << '\''; }
void trace(const char *x)                 { cerr << '\"' << x << '\"'; }
void trace(const string &x)               { cerr << '\"' << x << '\"'; }
void trace(bool x)                        { cerr << (x ? "true" : "false"); }

template <typename A, typename B>                         void trace(const pair <A, B> &x)         { cerr << '{'; trace(x.first); cerr << ','; trace(x.second); cerr << '}'; }
template <typename A, typename B, typename C>             void trace(const tuple <A, B, C> &x)     { cerr << '{'; trace(get <0> (x)); cerr << ','; trace(get <1> (x)); cerr << ','; trace(get <2> (x)); cerr << '}'; }
template <typename A, typename B, typename C, typename D> void trace(const tuple <A, B, C, D> &x)  { cerr << '{'; trace(get <0> (x)); cerr << ','; trace(get <1> (x)); cerr << ','; trace(get <2> (x)); cerr << ','; trace(get <3> (x)); cerr << '}'; }
template <typename T>                                     void trace(const T &x)                   { int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), trace(i); cerr << "}"; }
template <size_t N>                                       void trace(bitset <N> v)                 { cerr << '{'; for (size_t i = 0; i < N; i++) cerr << static_cast <char> ('0' + v[i]); cerr << '}'; }
void _print()                                                                                      { cerr << "]\n"; }
template <typename T, typename... V>                      void _print(T t, V... v)                 { trace(t); if (sizeof...(v)) cerr << ", "; _print(v...); }

#ifndef ONLINE_JUDGE
      #define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
      #define debug(x...)
#endif // ONLINE_JUDGE

template <typename T> using ordered_set                = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T1, typename T2> using ordered_map  = tree <T1, T2, less <T1>, rb_tree_tag, tree_order_statistics_node_update>;

template <class T> bool MIN(T &a, T b)                 { return a > b ? (a = b, true) : false; }
template <class T> bool MAX(T &a, T b)                 { return a < b ? (a = b, true) : false; }

template <class T> T binaryExpo(T a, T p)              { if (p == 0) { return 1LL; } if (p == 1) { return a; } if (p & 1) { return a * binaryExpo(a, p - 1); } T ret = binaryExpo(a, p / 2); return ret * ret; }
template <class T> T bigMod(T a, T p, T m)             { if (p == 0) { return 1LL % m; } if (p == 1) { return a % m; } if (p & 1) { return (a % m * bigMod(a, p - 1, m)) % m; } T ret = bigMod(a, p / 2, m) % m; return (ret * ret) % m; }
template <class T> T modInv(T a, T m)                  { return bigMod(a, m - 2, m); }

template <typename T> string num_str(T num)            { stringstream ss; ss << num; return ss.str(); }
long long str_num(string s)                            { long long num; istringstream iss(s); iss >> num; return num; }

inline long long ON(long long mask, int pos)           { return mask |= (1LL << pos); }
inline long long OFF(long long mask, int pos)          { return mask &= ~(1LL << pos); }
inline long long TOGGLE(long long mask, int pos)       { return mask ^= (1LL << pos); }
inline bool CHECK(long long mask, int pos)             { return (bool)(mask >> pos & 1LL); }

inline int ONE(long long mask)                         { return __builtin_popcountll(mask); }
inline int LZERO(long long mask)                       { return 31 - __builtin_clzll(mask); };
inline int TZERO(long long mask)                       { return __builtin_ctzll(mask); };

int dr4[] = {+1, -1, +0, +0};                       // 4 Direction - DOWN, UP, RIGHT, LEFT
int dc4[] = {+0, +0, +1, -1};
int dr8[] = {+0, +0, +1, -1, +1, +1, -1, -1};       // 8 Direction - RIGHT, LEFT, DOWN, UP, RIGHT-DOWN, LEFT-DOWN, UP-RIGHT, UP-LEFT
int dc8[] = {+1, -1, +0, +0, +1, -1, +1, -1};
int drk[] = {-1, +1, -2, -2, -1, +1, +2, +2};       // knight Moves
int dck[] = {-2, -2, -1, +1, +2, +2, +1, -1};

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

#define ll              long long int
#define ull             unsigned long long int
#define endl            '\n'
#define pii             pair <int, int>
#define pll             pair <ll, ll>
#define mk              make_pair
#define ff              first
#define ss              second
#define all(a)          (a).begin(), (a).end()
#define rall(a)         (a).rbegin(), (a).rend()
#define eb              emplace_back
#define pb              push_back
#define pf              push_front
#define allUpper(a)     transform(all(a), a.begin(), :: toupper)
#define allLower(a)     transform(all(a), a.begin(), :: tolower)
#define LINE            cerr << __LINE__ << " ";
#define memo(a, b)      memset(a, b, sizeof a)
#define sz(a)           (int)(a.size())
#define ook             order_of_key             // The number of items in a set that are strictly smaller than k
#define fbo             find_by_order            // It returns an iterator to the i'th largest element

#define int             long long int

const int maxn = 3e6 + 5;
const int logn = 19;
const long long MOD = 1e9 + 7;
const long long INF = 1e18;


signed main()
{
      ios_base::sync_with_stdio(false);
      cin.tie(nullptr); cout.tie(nullptr);
      cout.precision(12);

      // atcoder.jp
      if (fopen("in.txt", "r")) {
            freopen("in.txt", "r", stdin);
      }

      int n;
      cin >> n;
      string s;
      cin >> s;
      int red = count(all(s), 'R');
      int white = count(all(s), 'W');
      int mini = INF;
      MIN(mini, n - red);
      MIN(mini, n - white);
      int sum = 0;
      for (int i = 0; i < red; i++) {
            if (s[i] == 'W') {
                  sum++;
            }
      }
      MIN(mini, sum);
      cout << mini << endl;

      #ifndef ONLINE_JUDGE
            cerr << endl << "Time : " << 1.0 * clock() / CLOCKS_PER_SEC << " s." << endl;
      #endif // ONLINE_JUDGE

      return 0;
}


Measured memory_bytes: 3768