#include<bits/stdc++.h>
using namespace std;
#pragma GCC optimize ("Ofast")
typedef long long ll;
typedef pair<ll , ll> pll;
const ll maxn = 4e5 + 17;
ll n;
string s;
vector<ll> a , b;
ll ps[maxn] , c[maxn];
ll cal(ll k){
if(c[k] != -1) return c[k];
ll cur = 0;
for(ll t = 0 ; t < n ; t++){
ll i = a[t] , j = b[(t + k) % n];
if(i < j){
ll h1 = ps[j] - ps[i] , h0 = j - i - 1 - h1 , b0 = n - 1 - h0 , h = max(0ll , h1 - b0);
cur += j - i - 1 - (h << 1);
} else {
ll d = (n << 1) - (i - j + 1) , h1 = n - ps[i] + ps[j] , h0 = d - h1 , b0 = n - 1 - h0 , h = max(0ll , h1 - b0);
cur += d - (h << 1);
}
}
cur >>= 1;
return c[k] = cur;
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
memset(c , -1 , sizeof(c));
cin>>n>>s;
for(ll i = 0 ; i < (n << 1) ; i++){
if(s[i] == 'B'){
a.push_back(i);
} else {
b.push_back(i);
}
}
ps[0] = (s[0] == 'B');
for(ll i = 1 ; i < (n << 1) ; i++){
ps[i] = ps[i - 1] + (s[i] == 'B');
}
ll sq = 500;
ll res = 0;
for(ll i = 0 ; i < n ; i += sq){
c[i] = cal(i);
res = max(res , c[i]);
}
bool f = false;
for(ll i = sq ; i < n ; i += sq){
f |= (c[i - sq] < c[i]);
if(c[i - sq] >= c[i] && f){
for(ll j = max(0ll , i - (sq << 1)) ; j <= i ; j++){
res = max(res , cal(j));
}
cout<<res<<'\n';
return 0;
}
}
for(ll i = max(0ll , n - (sq << 1)) ; i < n ; i++){
res = max(res , cal(i));
}
for(ll i = 0 ; i < sq ; i++){
res = max(res , cal(i));
}
cout<<res<<'\n';
return 0;
}
