#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
string s;
cin >> s;
vector<int> b, w;
for(int i = 0; i < 2 * n; ++i) {
if(s[i] == 'B')
b.push_back(i + 1);
else
w.push_back(i + 1);
}
int prefb[4 * n + 1], prefw[4 * n + 1];
memset(prefb, 0, sizeof(prefb));
memset(prefw, 0, sizeof(prefw));
//cout << "TEST" << endl;
for(int i = 1; i <= 2 * n; ++i) {
prefb[i] = prefb[i - 1];
prefw[i] = prefw[i - 1];
if(binary_search(b.begin(), b.end(), i))
prefb[i]++;
else
prefw[i]++;
}
//cout << "PREF BUILD DONE" << endl;
//cout << b.size() << " " << w.size() << endl;
long long ans = 0;
for(int i = 0; i < n; ++i) {
// match b[i] dan w[i]
long long res = 0;
for(int j = 0; j < n; ++j) {
int l = b[j], r = w[(i + j) % n];
//cout << l << " " << r << endl;
// cari antara l dan r
// kalo misal r < l, maka + 2 * n
if(r < l)
swap(l, r);
// pref[2 * n] - pref[r] + pref[l] buat seg 1
// pref[r] - pref[l] buat seg 2
int b1 = prefb[2 * n] - prefb[r] + prefb[l - 1], w1 = prefw[2 * n] - prefw[r] + prefw[l - 1];
int b2 = prefb[r - 1] - prefb[l], w2 = prefw[r - 1] - prefw[l];
//cout << b1 << " " << b2 << " " << w1 << " " << w2 << endl;
res += min(b1, w2) + min(b2, w1);
}
//cout << res << " " << endl;
ans = max(ans, res);
}
//cout << endl;
cout << ans / 2 << endl;
}
