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

#include <bits/stdc++.h>
using namespace std;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

//========================TypeDefs===================================

typedef long long int ll;
typedef unsigned long long int ull;
typedef long double ldb;

typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
typedef pair<int, ll> pil;

//=========================MACROS====================================

#define pb push_back
#define popb pop_back()
#define pf push_front
#define popf pop_front()
#define si size()
#define be begin()
#define en end()
#define all(v) v.be, v.en
#define le length()
#define mp make_pair
#define mt make_tuple
#define acc(v) accumulate(all(v), 0)
#define fi first
#define se second

#define vi vector<int>
#define vll vector<ll>
#define vpll vector<pair<ll,ll>>
#define vvi vector<vector<int>>
#define vvll vector<vll>

#define mli map<ll,int>
#define mll map<ll,ll>

#define forz(i, n) for (int i = 0; i < n; i++)
#define fora(i, m, n) for (int i = m; i < n; i++)
#define rforz(i, n) for (int i = n - 1; i >= 0; i--)
#define rfora(i, n, m) for (int i = n; i >= m; i--)

#define deci(n) fixed << setprecision(n)
#define high(n) __builtin_popcount(n)
#define highll(n) __builtin_popcountll(n)
#define parity(n) __builtin_parity(n)
#define clz(n) __builtin_clz(n)
#define clzll(n) __builtin_clzll(n)
#define ctz(n) __builtin_ctz(n)

#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 lb lower_bound
#define ub upper_bound
#define er equal_range

#define findnot find_first_not_of

#define maxe *max_element
#define mine *min_element

#define MOD 1000000007
#define MOD2 998244353
#define PI 3.1415926535897932384
#define INF LLONG_MAX

#define gcd __gcd
#define fastio ios::sync_with_stdio(0), cin.tie(0), cout.tie(0)

#define endl "\n"
#define P0(a) cout << a << " "
#define P1(a) cout << a << endl
#define P2(a, b) cout << a << " " << b << endl
#define P3(a, b, c) cout << a << " " << b << " " << c << endl
#define P4(a, b, c, d) cout << a << " " << b << " " << c << " " << d << endl

#define oset tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define osetlli tree<lli, null_type, less<lli>, rb_tree_tag, tree_order_statistics_node_update>

#define ofk order_of_key
#define fbo find_by_order
//member functions :
//1. order_of_key(k) : number of elements sbtriectly lesser than k
//2. find_by_order(k) : k-th element in the set

//==============================FUNCTIONS===========================================

auto clk = clock();
mt19937_64 rang(chrono::high_resolution_clock::now().time_since_epoch().count());
void run_time()
{
#ifdef local
    cout << endl;
    cout << "Time elapsed: " << (double)(clock() - clk) / CLOCKS_PER_SEC << endl;
#endif
    return;
}

inline ll power(ll x, ll y, ll p = MOD)
{
    ll res = 1;
    x = x % p;
    while (y > 0)
    {
        if (y & 1)
            res = (res * x) % p;
        y = y >> 1;
        x = (x * x) % p;
    }
    return res;
}
void SieveOfEratosthenes(ll n) 
{ 
    // Create a boolean array "prime[0..n]" and initialize 
    // all entries it as true. A value in prime[i] will 
    // finally be false if i is Not a prime, else true. 
    bool prime[n+1]; 
    memset(prime, true, sizeof(prime)); 
  
    for (ll p=2; p*p<=n; p++) 
    { 
        // If prime[p] is not changed, then it is a prime 
        if (prime[p] == true) 
        { 
            // Update all multiples of p greater than or  
            // equal to the square of it 
            // numbers which are multiple of p and are 
            // less than p^2 are already been marked.  
            for (ll i=p*p; i<=n; i += p) 
                prime[i] = false; 
        } 
    } 
  
    // Print all prime numbers 
    for (ll p=2; p<=n; p++) 
       if (prime[p]) 
          cout << p << " "; 
}

inline ll modadd(ll a, ll b, ll m = MOD)
{
    a += b;
    if (a >= m)
        a -= m;
    return a;
}

inline ll modmul(ll a, ll b, ll m = MOD)
{
    return ((a % m) * (b % m)) % m;
}

inline ll modi(ll a, ll m = MOD) { return power(a, m - 2, m); }

long long int inverse(long long int i,ll m=MOD){
    if(i==1) return 1;
    return (m - ((m/i)*inverse(m%i))%m+m)%m;
}

bool isprime(ll n)
{
    if(n<2)
        return false;
    for(ll x:{2,3,5,7,11,13,17,19,23,29,31,37})
    {
        if(n==x)
            return true;
        bool flag=true;
        ll r=1;
        ll t=1;
        while(r<=((n-1)>>__builtin_ctzll(n-1)))
        {
            if(r&((n-1)>>__builtin_ctzll(n-1)))
                t=((__int128)t*x)%n;
            x=((__int128)x*x)%n;
            r<<=1;
        }
        if(t==1||t==n-1)
            flag=false;
        for(r=0;r<__builtin_ctzll(n-1);r++)
        {
            t=((__int128)t*t)%n;
            if(t==n-1)
                flag=false;
        }
        if(flag)
            return false;
    }
    return true;
}
//================================CODE=============================================

int main()
{
    fastio;
    ll n,r=0,w=0;
    cin>>n;
    string s;
    cin>>s;
    fora(i,0,n)
    {
        if(s[i]=='R')
        r++;
        else
        {
            w++;
        }
        
    }
    ll s1=0,s2=0;
    fora(i,0,n)
    {
        if(i<r&&s[i]=='W')
        {
            s1++;
        }
        if(i>=r&&s[i]=='R')
        {
            s2++;
        }
    }
    ll ans=max(s1,s2);
    cout<<ans<<endl;
    run_time();
    return 0;
}

Measured memory_bytes: 3756