#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
#define fi first
#define se second
#define mp make_pair
#define fastIO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
const int N = (int)4e5 + 100;
int col[N];
int n, m;
vector<int> C[2];
int segt[N*2];
void upd(int id, int v){
id += m;
while(id > 0){
segt[id] += v;
id /= 2;
}
}
int get(int l, int r){
l += m;
r += m;
int cum = 0;
while(l <= r){
if(l % 2 == 1) cum += segt[l ++ ];
if(r % 2 == 0) cum += segt[r -- ];
l /= 2;
r /= 2;
}
return cum;
}
ll f(int k){
vector<pii> seg;
for(int i = 0 ; i < n; i ++ ){
seg.push_back(mp(C[0][i], C[1][(i + k) % n]));
}
for(auto &x : seg){
if(x.fi > x.se) swap(x.fi, x.se);
}
for(int i = 0 ; i < m * 2; i ++ ){
segt[i]=0;
}
sort(seg.begin(), seg.end());
ll ff = 0;
for(auto q : seg){
ff += get(q.fi, q.se);
upd(q.se, +1);
}
return ff;
}
int main(){
fastIO;
cin >> n;
m = n * 2;
char qf;
for(int i = 0; i < m ; i ++ ){
cin >> qf;
if(qf == 'B'){
col[i]=0;
}
else{
col[i]=1;
}
C[col[i]].push_back(i);
}
ll high = f(0);
int li = 1;
int ri = n;
int mid;
while(li < ri){
mid = (li + ri) / 2;
if(f(mid) < f(mid + 1)){
li = mid + 1;
}
else{
ri = mid;
}
}
high = max(high, f(li));
cout << high << "\n";
return 0;
}
