#include<bits/stdc++.h>
using namespace std;
using ll=long long;
//#pragma GCC optimize("03,unroll-loops")
//#pragma GCC target("avx2")
//#pragma GCC target("sse4")
#define all(v) v.begin(),v.end()
#define F first
#define S second
#define pub push_back
#define pob pop_back
#define puf push_front
#define pof pop_front
//#define randi uniform_int_distribution<long long>
#define damoon(v) v.resize(unique(all(v))-v.begin())
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//randi dist(0,10000000000000000);
typedef pair<int,int> pii;
typedef pair<long long,long long> pll;
typedef pair<int,bool> pib;
typedef pair<long long,bool> plb;
typedef pair<int,pii> pip;
typedef pair<pii,int> ppi;
typedef vector<int> veci;
typedef vector<long long> vecl;
typedef vector<bool> vecb;
typedef vector<pii> vecp;
typedef set<int> seti;
typedef set<long long> setl;
typedef set<pii> setp;
typedef map<int,int> mapii;
typedef map<long long,long long> mapll;
typedef map<int,bool> mapib;
typedef map<long long,bool> maplb;
const int inf=1e9,mod=1e9+7,neginf=-1e9,N=4e5+5;
const double PI=acos(-1);
struct Fenwick
{
veci f;
int n;
Fenwick(int m)
{
n=m;
f.assign(n+1,0);
}
void update(int i,int v)
{
for(++i;i<=n;i+=i&-i)
f[i]+=v;
}
int get(int i)
{
int s=0;
for(++i;i>0;i-=i&-i)
s+=f[i];
return s;
}
};
int n;
ll krj[N];
veci b,w;
inline ll res(int i,Fenwick &Fen)
{
if(krj[i]!=-1)
return krj[i];
vecp v;
for(int j=0;j<n;j++)
v.pub(pii(min(b[j],w[(j+i)%n]),max(b[j],w[(j+i)%n])));
sort(all(v),greater<>());
ll ret=0;
for(int j=0;j<n;j++)
{
ret+=Fen.get(v[j].S);
Fen.update(v[j].F+0,+1);
Fen.update(v[j].S+1,-1);
}
for(int j=0;j<n;j++)
{
Fen.update(v[j].F+0,-1);
Fen.update(v[j].S+1,+1);
}
return krj[i]=ret;
}
void solve()
{
cin>>n;
for(int i=0;i<2*n;i++)
{
char c;
cin>>c;
if(c=='B')
b.pub(i);
else
w.pub(i);
}
memset(krj,-1,sizeof(krj));
Fenwick Fen(N);
int L=0,R=n;
while(L<R)
{
int mid=(L+R)>>1;
if(res(mid,Fen)<res(0,Fen))
{
if(res(0,Fen)>res(1,Fen))
{
L=mid+1;
}
else
{
R=mid-1;
}
}
else
{
if(res(mid,Fen)<res(mid+1,Fen))
{
L=mid+1;
}
else
{
R=mid-1;
}
}
}
cout<<res(L,Fen)<<"\n";
}
int main()
{
cin.tie(0);
cout.tie(0);
ios_base::sync_with_stdio(false);
//ifstream fin("in.txt");
//ofstream fout("out.txt");
int t=1;
//cin>>t;
while(t--)
{
solve();
}
}
