#include<bits/stdc++.h>
int main() {
std::cin.tie(nullptr)->sync_with_stdio(false);
int n;
std::cin >> n;
std::string s;
std::cin >> s;
std::vector<int> wh, bl;
for (int i = 0; i < s.size(); ++i) {
if (s[i] == 'W') wh.push_back(i);
else bl.push_back(i);
}
auto solve = [&](int k) -> long long {
long long cur = 0;
for (int i = 0; i < n; ++i) {
cur += n - std::abs(n - std::abs(bl[(i + k) % n] - wh[i])) - 1;
}
return cur;
};
int p = 0;
for (int k = 1 << 20; k; k >>= 1) {
if (solve(p) < solve((p + k) % n)) p = (p + k) % n;
if (solve(p) < solve((p - k % n + n) % n)) p = (p - k % n + n) % n;
}
std::cout << solve(p) / 2;
}
