#include <bits/stdc++.h>
#define int long long
#define mp make_pair
#define pb push_back
#define ld long double
#define pii pair<int,int>
#define sz(x) (int)x.size()
#define piii pair<pii,pii>
#define precise cout<<fixed<<setprecision(10)
#define st first
#define nd second
#define ins insert
#define vi vector<int>
#define BOOST ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
using namespace std;
const int MAX=2e5+5;
vi b,w;
char s[MAX*2];
int ans=0;
int n;
int ile(int x,int y){
if (x>y)swap(x,y);
int between=y-x-1;
return min(n*2-2-between,between);
}
int f(int shift){
int sum=0;
assert(sz(b)==sz(w) && sz(b)==n);
for (int i=0;i<n;i++){
int x=b[i];
int y=w[(i+shift)%n];
//if (shift==1)cout<<"TERAZ "<<x<<" "<<y<<" "<<ile(x,y)<<"\n";
sum+=ile(x,y);
}
return sum/2;
}
int32_t main(){
BOOST;
cin>>n;
for (int i=0;i<n*2;i++){
cin>>s[i];
if (s[i]=='B')b.pb(i);
else w.pb(i);
}
for (int i=0;i<=n-1;i++){
ans=max(ans,f(i));
}
cout<<ans;
return 0;
}
