/**
____ ____ ____ ____ ____
||a |||t |||o |||d |||o ||
||__|||__|||__|||__|||__||
|/__\|/__\|/__\|/__\|/__\|
**/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N_MAX = 200000;
int N;
string s;
vector <int> v[2];
int Fen[N_MAX * 2 + 2];
void update (int x) {
x++;
for (int i = x; i <= N * 2; i += i & -i) {
Fen[i]++;
}
}
int query (int x) {
x++;
int cnt = 0;
for (int i = x; i >= 1; i -= i & -i) {
cnt += Fen[i];
}
return cnt;
}
int intersections (int x, int y) {
return query(y) - query(x);
}
ll solve (int offs) {
for (int i = 1; i <= N * 2; i++) {
Fen[i] = 0;
}
vector <pair <int, int>> segments;
for (int i = 0; i < N; i++) {
int j = (i + offs) % N;
segments.push_back(make_pair(v[0][i], v[1][j]));
if (segments.back().first > segments.back().second) {
swap(segments.back().first, segments.back().second);
}
}
sort(segments.begin(), segments.end());
ll cnt = 0;
for (pair <int, int> p : segments) {
cnt += intersections(p.first, p.second);
update(p.second);
}
return cnt;
}
int main () {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> N;
cin >> s;
for (int i = 0; i < N * 2; i++) {
v[(s[i] == 'B')].push_back(i);
}
int l = 0, r = N - 1;
while (l < r) {
int mid = (l + r) / 2;
if (solve(mid) < solve(mid + 1)) {
l = mid + 1;
} else {
r = mid;
}
}
cout << solve(l) << "\n";
return 0;
}
