#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define int ll
#define ld long double
#define pii pair<int, int>
#define f first
#define s second
#define boost() cin.tie(0), cin.sync_with_stdio(0)
const int MN = 400005;
int n;
string s;
vector<int> a, b;
int calc(int x) {
int ret = 0;
for (int i = 0; i < n; i++) {
int l = a[i], r = b[(i + x) % n];
if (l > r) swap(l, r);
int p1 = r - l - 1, p2 = 2 * n - p1 - 2;
ret += min(p1, p2);
}
return ret / 2;
}
int32_t main() {
boost();
cin >> n >> s;
for (int i = 0; i < 2 * n; i++) {
if (s[i] == 'B') a.push_back(i);
else b.push_back(i);
}
int lo = 0, hi = n - 1;
while (lo < hi) {
int mid = (lo + hi) >> 1;
if (calc(mid) < calc(mid + 1)) lo = mid + 1;
else hi = mid;
}
printf("%lld\n", calc(lo));
return 0;
}
