#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5 + 5;
int a[2*N];
ll dp[N];
ll solve(int n, int i) {
if (dp[i] != 0)
return dp[i];
int ptr[2] = {i+n, i+n}, cnt[4] = {0, 0, 0, 0};
// auto next = [&](int& j) {
// j = j+1 < 2*n ? j+1 : j+1-2*n;
// };
#define next(x) x = x+1 < 2*n ? x+1 : x+1-2*n
ll val = 0;
for (int j = i, c = a[i]; j < i+n; c = a[++j]) {
while (ptr[c] != i && a[ptr[c]] == c)
cnt[c+2]++, next(ptr[c]);
next(ptr[c]);
val += min(cnt[c+2], cnt[c^1]) + cnt[c]++;
}
return dp[i] = val;
}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int get(int l, int r) {
return uniform_int_distribution<int>(l, r)(rng);
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0);
auto cur_t = clock();
int n; cin >> n;
string s; cin >> s;
for (int i = 0; i < 2*n; i++)
a[i] = s[i] == 'B';
ll ans = 0;
while (clock() - cur_t < 1950) {
ans = max(ans, solve(n, get(0, n-1)));
// cerr << "HUH" << endl;
}
cout << ans << "\n";
}
