#include <iostream>
#include <stdio.h>
#include <cmath>
#include <algorithm>
#include <climits>
#include <vector>
#include <iomanip>
#include <string>
#include <queue>
#include <numeric>
#include <functional>
#include <array>
#include <map>
#include <set>

#define INF 1000000007 //1000000000000000003
#define MOD 1000000007
#define MAX 2000005

#define ll long long

//printf("%.10f", );
//cout << fixed << setprecision(10) << heikin << endl;
//vector<vector<int>> data(3, vector<int>(4, -1));
//left + 1 < top

using namespace std;
using P = pair<ll, ll>;
using Pc = pair<double, int>;
using T = tuple<int, int, int>;
using edge = struct { int to; long long dist; };
const double PI = 3.141592653589793;
long long fact[MAX], inv[MAX];
int dx[] = { -1,0,1,0 };
int dy[] = { 0,-1,0,1 };

//////////////////////////////////

vector<vector<int>> bingo(3, vector<int>(3));
vector<vector<int>> tree(100005);
int color[100005];
map<P, int>colorMap;
int m = 0;

struct Connection {
	vector<int>connect;
	int parent = INF;
	int dep = INF;
	Connection() {}
};

bool compare_by_second(pair<int, int> a, pair<int, int> b) {
	if (a.second != b.second) {
		return a.second < b.second;
	}
	else {
		return a.first < b.first;
	}
}

struct UnionFind {
	vector<int>uf;

	UnionFind(int size) :uf(size, -1) {};

	int root(int target) {
		if (uf[target] < 0)return target;
		else return uf[target] = root(uf[target]);
	}

	void merge(int a, int b) {
		a = root(a);
		b = root(b);
		if (a == b)return;
		if (uf[b] < uf[a])swap(a, b);
		uf[a] += uf[b];
		uf[b] = a;
	}
};

ll CalcPow(ll num, ll power) {
	if (power == 0)return 1;
	if (power % 2 == 0) {
		ll half = CalcPow(num, power / 2);
		return (half * half) % MOD;
	}
	else return (num * CalcPow(num, power - 1)) % MOD;
}

ll combination(ll all, ll select) {
	if (all < select) return 0;
	if (all < 0 || select < 0) return 0;
	return fact[all] * (inv[select] * inv[all - select] % MOD) % MOD;
}

void initCombination(int max) {
	fact[0] = fact[1] = 1;
	for (int i = 2; i <= max; ++i) {
		fact[i] = fact[i - 1] * i % MOD;
	}

	inv[max] = CalcPow(fact[max], MOD - 2);
	for (int i = max - 1; 0 <= i; --i) {
		inv[i] = (inv[i + 1] * (i + 1)) % MOD;
	}
}

ll getGcd(ll a, ll b) {
	if (a < b) {
		ll temp = b;
		b = a;
		a = temp;
	}

	ll r;
	while ((r = a % b) != 0) {
		a = b;
		b = r;
	}
	return b;
}

/*int getNum(ll target) {

int bottom = 0;
int top = 151;
while (1 < top - bottom) {
int middle = (top + bottom) / 2;
if (target < line[middle])top = middle;
else bottom = middle;
}
return bottom;
}*/

/*
void dfs(int depth, int next, int parent) {
++depth;
for (int c : connection[next].connect) {
if (c == parent)continue;
if (depth < connection[c].dep) {
if (connection[c].dep == INF)co++;
connection[c].dep = depth;
connection[c].parent = next;
}
else if (connection[c].parent == next) {
continue;
}
dfs(depth, c, next);
}
}*/
/*
int N, M, Q;
int mx = 0;
int abcd[55][4];

void calc(vector<int> A, int current){
if (A.size() == N) {
int sum = 0;
for (int i = 0; i < Q; ++i) {
if (A[abcd[i][1]] - A[abcd[i][0]] == abcd[i][2])sum += abcd[i][3];
}
mx = max(mx, sum);
return;
}
for (int i = current; i <= M; ++i) {
A.push_back(i);
calc(A, i);
A.pop_back();
}
}*/

enum Color {
	Red,
	Green,
	None,
};

using PC = pair<int, Color>;

string getString(ll num) {
	int iNum = (int)num;
	//1->a , 26->z
	if (num == 0)iNum += 26;
	iNum += 96;
	char c = int(iNum);
	string rS = "c";
	rS[0] = c;
	return rS;
}

int solve(int current, int x, int n) {
	if (n == 1)return 1;
	if (x < (n * current))return current;
	solve(n * current, x, n);
}

int main() {

	int N; cin >> N;
	string C; cin >> C;

	int redCount = 0;
	int wIndex = -1;
	int ans = 0;
	for (int i = C.size() - 1; 0 <= i; --i) {
		if (C[i] == 'R') ++redCount;
		else if(redCount <= 0)wIndex = i;
	}

	for (int i = 0; i < N; ++i) {
		if (0 < redCount) {
			if (C[i] == 'W') {
				//交換を優先する
				if (wIndex == -1) {
					C[C.size() - 1] = 'W';
					C[i] = 'R';
					wIndex = C.size() - 1;
				}
				else {
					C[wIndex--] = 'W';
					C[i] = 'R';
				}
				++ans;
				--redCount;
			}
			else {
				--redCount;
			}
		}
	}

	cout << ans << endl;
}
	




Measured memory_bytes: 5972