//HEADER FILES AND NAMESPACES

#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>  
#include <ext/pb_ds/tree_policy.hpp>   
using namespace std;
using namespace __gnu_pbds;

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

template <typename T>
using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag, tree_order_statistics_node_update>; 

// DEFINE STATEMENTS
const long long infty = 1e18;
#define num1 1000000007
#define num2 998244353
#define REP(i,a,n) for(ll i=a;i<n;i++)
#define REPd(i,a,n) for(ll i=a; i>=n; i--)
#define pb push_back
#define pob pop_back
#define f first
#define s second
#define fix(f,n) std::fixed<<std::setprecision(n)<<f
#define all(x) x.begin(), x.end()
#define M_PI 3.14159265358979323846
#define epsilon (double)(0.000000001)

typedef long long ll;
typedef vector<long long> vll;
typedef pair<long long, long long> pll;
typedef vector<pair<long long, long long>> vpll;
typedef vector<int> vii;

// DEBUG FUNCTIONS 
#ifndef ONLINE_JUDGE

template<typename T>
void __p(T a) {
	cout<<a;
}
template<typename T, typename F>
void __p(pair<T, F> a) {
	cout<<"{";
	__p(a.first);
	cout<<",";
	__p(a.second);
	cout<<"}";
}
template<typename T>
void __p(std::vector<T> a) {
	cout<<"{";
	for(auto it=a.begin(); it<a.end(); it++)
		__p(*it),cout<<",}"[it+1==a.end()];
}
template<typename T>
void __p(std::set<T> a) {
	cout<<"{";
	for(auto it=a.begin(); it!=a.end();){
		__p(*it); 
		cout<<",}"[++it==a.end()];
	}

}
template<typename T>
void __p(std::multiset<T> a) {
	cout<<"{";
	for(auto it=a.begin(); it!=a.end();){
		__p(*it); 
		cout<<",}"[++it==a.end()];
	}
}
template<typename T, typename F>
void __p(std::map<T,F> a) {
	cout<<"{\n";
	for(auto it=a.begin(); it!=a.end();++it)
	{
		__p(it->first);
		cout << ": ";
		__p(it->second);
		cout<<"\n";
	}
	cout << "}\n";
}

template<typename T, typename ...Arg>
void __p(T a1, Arg ...a) {
	__p(a1);
	__p(a...);
}
template<typename Arg1>
void __f(const char *name, Arg1 &&arg1) {
	cout<<name<<" : ";
	__p(arg1);
	cout<<endl;
}
template<typename Arg1, typename ... Args>
void __f(const char *names, Arg1 &&arg1, Args &&... args) {
	int bracket=0,i=0;
	for(;; i++)
		if(names[i]==','&&bracket==0)
			break;
		else if(names[i]=='(')
			bracket++;
		else if(names[i]==')')
			bracket--;
	const char *comma=names+i;
	cout.write(names,comma-names)<<" : ";
	__p(arg1);
	cout<<" | ";
	__f(comma+1,args...);
}
#define trace(...) cout<<"Line:"<<__LINE__<<" ", __f(#__VA_ARGS__, __VA_ARGS__)
#else
#define trace(...)
#define error(...)
#endif

// DEBUG FUNCTIONS END 

// CUSTOM HASH TO SPEED UP UNORDERED MAP AND TO AVOID FORCED CLASHES
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }
 
    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};

mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());  // FOR RANDOM NUMBER GENERATION

ll mod_exp(ll a, ll b, ll c)
{
	ll res=1;   a=a%c;
	while(b>0)
	{
		if(b%2==1)
			res=(res*a)%c;
		b/=2;
		a=(a*a)%c;
	}
	return res;
}
ll mymod(ll a,ll b)
{
		return ((a%b)+b)%b;
}
ll gcdExtended(ll,ll,ll *,ll *);
ll modInverse(ll a, ll m) 
{ 
		ll x, y; 
		ll g = gcdExtended(a, m, &x, &y); 
		g++;   //this line was added just to remove compiler warning
		ll res = (x%m + m) % m;
		return res;
} 
ll gcdExtended(ll a, ll b, ll *x, ll *y) 
{ 
		if (a == 0) 
		{ 
				*x = 0, *y = 1; 
				return b; 
		} 
		ll x1, y1; 
		ll gcd = gcdExtended(b%a, a, &x1, &y1);
		*x = y1 - (b/a) * x1; 
		*y = x1; 
		return gcd; 
}
struct node
{
	vll adj;
};
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	ll n;
	cin>>n;
	string s;
	cin>>s;
	ll en = n-1, st=0;
	ll cnt=0;
	while(st<en)
	{
		while(s[st]!='W' && st<en)
		{
			st++;
		}
		while(s[en]!='R' && en>st)
		{
			en--;
		}
		if(en<=st)
		{
			break;
		}
		else
		{
			cnt++;
			swap(s[st], s[en]);
			st++;
			en--;
		}
	}
	cout << cnt << "\n";
	return 0;
}



Measured memory_bytes: 3736