#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5 + 5;
int a[4*N], nxt[3*N][2];
ll solve(int n, int i) {
int ptr[2] = {i+n, i+n}, cnt[4] = {0, 0, 0, 0};
ll val = 0;
for (int j = i, c = a[i]; j < i+n; c = a[++j]) {
cnt[c+2] += nxt[ptr[c]][c] - ptr[c];
ptr[c] = nxt[ptr[c]][c] + 1;
val += min(cnt[c+2], cnt[c^1]) + cnt[c]++;
}
return val;
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0);
int n; cin >> n;
string s; cin >> s;
for (int i = 0; i < 2*n; i++)
a[i] = a[i+2*n] = s[i] == 'B';
nxt[3*n][0] = nxt[3*n][1] = 3*n;
for (int i = 3*n-1; i >= 0; i--) {
nxt[i][0] = nxt[i+1][0], nxt[i][1] = nxt[i+1][1];
nxt[i][a[i]^1] = i;
}
ll ans = solve(n, 0);
for (int i = 1; i < n; i++) if (s[i] == s[i+n])
ans = max(ans, solve(n, i));
cout << ans << "\n";
}
