/**
____ ____ ____ ____ ____
||a |||t |||o |||d |||o ||
||__|||__|||__|||__|||__||
|/__\|/__\|/__\|/__\|/__\|
**/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int N;
string s;
vector <int> v[2];
bool intersect (int a1, int a2, int b1, int b2) {
if (a1 > a2) {
swap(a1, a2);
}
if (b1 > b2) {
swap(b1, b2);
}
if (a1 < b1 && b1 < a2 && a2 < b2) {
return true;
}
if (b1 < a1 && a1 < b2 && b2 < a2) {
return true;
}
return false;
}
int solve (int offs) {
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]));
}
int cnt = 0;
for (int i = 0; i < (int) segments.size(); i++) {
for (int j = i + 1; j < (int) segments.size(); j++) {
cnt += intersect(segments[i].first, segments[i].second,
segments[j].first, segments[j].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;
}
