#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int N;
cin >> N;
vector<int> W, B;
W.reserve(N);
B.reserve(N);
for (int i = 0; i < 2 * N; ++i) {
char c;
cin >> c;
(c == 'W' ? W : B).push_back(i);
}
auto f = [&](int k) {
ll ret = 0;
for (int i = 0; i < N; ++i) {
int j = i + k;
if (j >= N) j -= N;
int d = abs(W[i] - B[j]);
ret += min(d - 1, 2 * N - 1 - d);
}
return ret / 2;
};
int min = 0, max = N - 1;
int s = 1, t = 2;
while (t < max - max + 2) swap(s += t, t);
int a = min - 1, b = min + t - 1, x = min + t - s - 1;
ll fx = f(x), fy;
while (a + b != 2 * x) {
int y = a + b - x;
if (y > max || fx > (fy = f(y))) {
b = a;
a = y;
} else {
a = x;
x = y;
fx = fy;
}
}
cout << fx << '\n';
return 0;
}
