#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp>
#define ll long long int
#define ld long double
#define pb push_back
#define pll pair<ll, ll> 
#define tri pair<pll, ll>
#define vl vector<ll> 
#define vvl vector< vector<ll> > 
#define vlp vector< pair<ll, ll> >
#define vllp vector<pair<pll, ll> > 
#define mll map<ll, ll> 
#define rep(i,a)  for(ll i=0; i< a; i++)
#define rep1(i,a)   for(ll i = 1; i< a; i++)
#define foi(i, a, b)    for(ll i = a; i<b ; i++)
#define fod(i, a, b)    for(ll i = a; i>=b ; i--)
#define mp make_pair
#define all(v)  (v).begin(), (v).end()
#define fst first
#define sec second
#define ff first.first
#define fs first.second
#define max3(a, b, c)   max(max(a, b), c)
#define min3(a, b, c)   min(min(a, b), c)
#define MAX 1000005
#define MOD 1000000007
// #define MOD 998244353
#define endl "\n"
#define INF (ll)1e18
#define s(v) (ll)v.size()
#define e(v) v.empty()
#define bscount(x) __builtin_popcountll(x)
// #define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>

using namespace std;
// using namespace __gnu_pbds;

void __print(ll x) {cerr << x;}
void __print(ld x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
 
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif

ll gcd(ll a, ll b){if(b==0)return a;return gcd(b, a%b);}
ll cpow(ll a, ll b, ll M){ll ans = 1;while(b){if(b&1) ans = ans*a%M; b/=2;a=a*a%M;}return ans;}
void ingraph(vvl& graph, ll m){ll x, y;rep(i, m){cin>>x>>y;x--, y--;graph[x].pb(y);graph[y].pb(x);}}
ll modify(ll n){ll res = n;res%=MOD;res+=MOD;res%=MOD;return res;}
ll fexp(ll a, ll b) {return cpow(a, b, MOD);}
ll cinv(ll a, ll p){return cpow(a, p-2, p);} 
ll inverse(ll a)    {return cinv(a, MOD);}

int main(){
	// #ifndef ONLINE_JUDGE
 //        freopen("../input.txt", "r", stdin);
 //        freopen("../output.txt", "w", stdout);
 //    #endif
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    ll t;
    // cin>>t;
    t = 1;
    ll tc = 0;
    while(tc++ < t){
        ll n;cin>>n;
        string str;cin>>str;
        set<ll> red, wh;
        rep(i, n){
            if(str[i] == 'R')   red.insert(i);
            else wh.insert(i);
        }
        ll ans = 0;
        while(1){
            if(e(red) || e(wh)) break;
            if(*(red.rbegin()) < *(wh.begin())) break;
            ll x = *(red.rbegin());
            ll y = *(wh.begin());
            red.erase(x);
            wh.erase(y);
            red.insert(y);
            wh.insert(x);
            ans++;
        }
        cout<<ans;
    }
}

Measured memory_bytes: 12992