#include <bits/stdc++.h>
#define fst first
#define snd second
#define pb push_back
#define SZ(x) (int)x.size()
#define ALL(x) x.begin(),x.end()
#define forn(i,a,b) for(int i = a; i<b; i++)
#define mset(a,v) memset(a,v,sizeof(a))
using namespace std;
typedef long long ll;
ll n;
struct STree{
ll n;
vector<ll> st;
STree(ll n):n(n),st(4*n+5) {}
void upd(ll k , ll l, ll r, ll p, ll v){
if(l+1==r){ st[k]=v; return; }
ll mid = (l+r)/2;
if(mid>p) upd(2*k,l,mid,p,v);
else upd(2*k+1,mid,r,p,v);
st[k]=st[2*k]+st[2*k+1];
}
ll qry(ll k, ll l, ll r, ll L, ll R){
if(l>=R || r<=L) return 0;
if(l>=L && r<=R) return st[k];
ll mid = (l+r)/2;
return qry(2*k,l,mid,L,R)+qry(2*k+1,mid,r,L,R);
}
void upd(ll p, ll v){ upd(1,0,n,p,v); }
ll qry(ll l, ll r){ return qry(1,0,n,l,r); }
};
vector<ll> b;
vector<ll> w;
ll solve(ll k){
ll cnt = 0;
vector<pair<pair<ll,ll>,ll>> line;
forn(i,0,SZ(b)){
ll j=k;
line.pb({{b[i],w[j]},0});
k++;
k%=n;
}
forn(i,0,SZ(line)){
line[i].snd=i;
if(line[i].fst.fst>line[i].fst.snd)
swap(line[i].fst.fst,line[i].fst.snd);
}
sort(ALL(line));
vector<pair<ll,ll>> inp; forn(i,0,SZ(line)) inp.pb({line[i].fst.fst, i});
vector<pair<ll,ll>> out; forn(i,0,SZ(line)) out.pb({line[i].fst.snd, i});
sort(ALL(inp)); reverse(ALL(inp));
sort(ALL(out)); reverse(ALL(out));
STree st(2*n); cnt=0;
forn(i,0,2*n){
while(!inp.empty() && inp.back().fst<=i){
st.upd(inp.back().fst,1);
inp.pop_back();
}
while(!out.empty() && out.back().fst<=i){
cnt+=st.qry(line[out.back().snd].fst.fst+1, line[out.back().snd].fst.snd+1);
st.upd(line[out.back().snd].fst.fst,(ll)0);
out.pop_back();
}
}
return cnt;
}
int main(){
cin>>n;
string s; cin>>s;
forn(i,0,2*n){
if(s[i]=='B'){
b.pb(i);
}else{
w.pb(i);
}
}
ll res = 0;
ll l = 0; ll r=n-1;
while(l<=r){
ll mid1 = l+(r-l)/3;
ll mid2 = r-(r-l)/3;
ll c1 = solve(mid1);
ll c2 = solve(mid2);
if(c1<c2){
l=mid1+1;
}else{
r=mid2-1;
}
res=max(res,max(c1,c2));
}
cout<<res<<'\n';
return 0;
}
