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

#define ALL(obj) (obj).begin(),(obj).end()
#define SPEED cin.tie(0);ios::sync_with_stdio(false);

template<class T> using PQ = priority_queue<T>;
template<class T> using PQR = priority_queue<T,vector<T>,greater<T>>;

constexpr long long MOD = (long long)1e9 + 7;
constexpr long long MOD2 = 998244353;
constexpr long long HIGHINF = (long long)1e18;
constexpr long long LOWINF = (long long)1e15;
constexpr long double PI = 3.1415926535897932384626433L;

template <class T> vector<T> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
std::vector<std::string> split(const std::string &str, const char delemiter) {std::vector<std::string> res;std::stringstream ss(str);std::string buffer; while( std::getline(ss, buffer, delemiter) ) res.push_back(buffer); return res;}
int msb(int x) {return x?31-__builtin_clz(x):-1;}
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}

/*
 * @title DisjointSparseTable
 */
template<class Operator> class DisjointSparseTable{
public:
	using TypeNode = typename Operator::TypeNode;
	size_t depth;
	size_t length;
	vector<TypeNode> node;
	vector<size_t> msb;

	DisjointSparseTable(const vector<TypeNode>& vec) {
		for(depth = 0;(1<<depth)<=vec.size();++depth);
		length = (1<<depth);
		
		//msb
		msb.resize(length,0);
		for(int i = 0; i < length; ++i) for(int j = 0; j < depth; ++j) if(i>>j) msb[i] = j;

		//init value
		node.resize(depth*length,Operator::unit_node);
		for(int i = 0; i < vec.size(); ++i) node[i] = vec[i];

		for(int i = 1; i < depth; ++i) {
			for(int r = (1<<i),l = r-1; r < length; r += (2<<i),l = r-1){
				//init accumulate
				node[i*length+l] = node[l];
				node[i*length+r] = node[r];
				//accumulate
				for(int k = 1; k < (1<<i); ++k) {
					node[i*length+l-k] = Operator::func_node(node[i*length+l-k+1],node[l-k]);
					node[i*length+r+k] = Operator::func_node(node[i*length+r+k-1],node[r+k]);
				}
			}
		}
	}

	//[l,r)
	TypeNode get(int l,int r) {
		r--;
		return (l>r||l<0||length<=r) ? Operator::unit_node: (l==r ? node[l] : Operator::func_node(node[msb[l^r]*length+l],node[msb[l^r]*length+r]));
	}
};

//sum
template<class T> struct NodeSum {
	using TypeNode = T;
	inline static constexpr TypeNode unit_node = 0;
	inline static constexpr TypeNode func_node(TypeNode l,TypeNode r){return l+r;}
};

int main() {
    int N; cin >> N;
    string S; cin >> S;
    int ans=N;
    vector<int> r(N,0),w(N,0);
    for(int i = 0; i < N; ++i){
        (S[i]=='R'?r:w)[i]=1;
    }
    DisjointSparseTable<NodeSum<int>> rsum(r),wsum(w);
    chmin(ans,rsum.get(0,N));
    chmin(ans,wsum.get(0,N));
    for(int i = 0; i+1 < N; ++i){
        int cr=wsum.get(0,i+1);
        int cw=rsum.get(i+1,N);
        int m = min(cr,cw);
        cr -= m;
        cw -= m;
        chmin(ans,m+max(cr,cw));
    }
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 46104