#include "bits/stdc++.h"
using namespace std;
#ifdef Nero
#include "Deb.h"
#else
#define debug(...)
#endif
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<int> black, white;
for (int i = 1; i <= n * 2; ++i) {
char c;
cin >> c;
if (c == 'B') {
black.push_back(i);
} else {
white.push_back(i);
}
}
auto f = [&](int j) {
long long ret = 0;
for (int i = 0; i < n; ++i) {
int x = white[i];
int y = black[(j + i) % n];
ret += min(abs(x - y), 2 * n - abs(x - y)) - 1;
}
return ret;
};
int l = 0, r = n - 1;
while (l < r) {
int mid = (l + r) / 2;
long long c1 = f(mid), c2 = f(mid + 1);
if (c1 < c2) {
l = mid + 1;
} else {
r = mid;
}
}
cout << f(l) / 2 << '\n';
return 0;
}
