#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5 + 5;
int a[4*N], nxt[3*N][2];
int idx[4*N][2], vis[2*N];
vector<int> pos;
ll solve(int n, int i) {
int ptr[2] = {i+n, i+n};
ll val = 0;
for (int j = i; j < i+n; ++j)
val += min((ptr[a[j]] = nxt[ptr[a[j]]][a[j]] + 1) - n - i - 1, j - i);
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;
}
idx[0][a[0]] = 1, idx[0][a[0]^1] = 0;
for (int i = 1; i < 4*n; i++) {
idx[i][0] = idx[i-1][0], idx[i][1] = idx[i-1][1];
idx[i][a[i]]++;
}
auto bin = [&](ll& ans) {
int lo = 0, hi = pos.size() - 1;
while (lo <= hi) {
int mi = (lo + hi) / 2;
ll x1 = solve(n, pos[mi]);
ans = max(ans, x1);
if (lo == hi)
break;
if (x1 < solve(n, pos[mi+1]))
lo = mi + 1;
else
hi = mi - 1;
}
// for (int i = max(0, hi - 20); i <= min((int)pos.size() - 1, lo + 20); i++)
// ans = max(ans, solve(n, pos[i]));
};
for (int i = n-1; i >= 0; i--) if (s[i] == s[i+n])
if (a[i] == 0)
vis[idx[nxt[i+n][a[i]]][a[i]^1] - idx[i][a[i]]] = i+1;
ll ans = solve(n, 0);
bin(ans);
for (int i: pos)
ans = max(ans, solve(n, i));
pos.clear();
for (int i = n-1; i >= 0; i--) if (s[i] == s[i+n])
if (a[i] == 1)
vis[idx[nxt[i+n][a[i]^1]][a[i]] - idx[nxt[i][a[i]]][a[i]^1]] = i+1;
for (int i = 0; i < n; i++) if (vis[i])
pos.push_back(vis[i] - 1);
bin(ans);
cout << ans << "\n";
}
