/***Farhan132***/
#include <bits/stdc++.h>
using namespace std;
typedef int ll;
typedef unsigned long long ull;
typedef double dd;
typedef vector<ll> vll;
typedef pair<ll , ll> ii;
typedef vector< ii > vi;
typedef pair < pair < ll , ll > , pair < ll , ll > > cm;
#define ff first
#define ss second
#define pb push_back
#define in insert
#define f0(b) for(int i=0;i<(b);i++)
#define f00(b) for(int j=0;j<(b);j++)
#define f1(b) for(int i=1;i<=(b);i++)
#define f11(b) for(int j=1;j<=(b);j++)
#define f2(a,b) for(int i=(a);i<=(b);i++)
#define f22(a,b) for(int j=(a);j<=(b);j++)
#define sf(a) scanf("%lld",&a)
#define sff(a,b) scanf("%lld %lld", &a , &b)
#define pf(a) printf("%lld\n",a)
#define pff(a,b) printf("%lld %lld\n", a , b)
#define PI 3.14159265359
#define bug printf("**!\n")
#define mem(a , b) memset(a, b ,sizeof(a))
#define front_zero(n) __builtin_clzll(n)
#define back_zero(n) __builtin_ctzll(n)
#define total_one(n) __builtin_popcountll(n)
#define clean fflush(stdout)
const ll mod = (ll)1e9 + 7;
const ll maxn = (ll)4e5 + 5;
const int nnn = 1048590;
const int inf = numeric_limits<int>::max()-1;
//const ll INF = numeric_limits<ll>::max()-1;
//const ll INF = 1e18;
ll dx[]={0,1,0,-1};
ll dy[]={1,0,-1,0};
ll dxx[]={0,1,0,-1,1,1,-1,-1};
ll dyy[]={1,0,-1,0,1,-1,1,-1};
/**
===========
Lost + something = something
Same + Same = Lost
i + j = i ^ j
===========
**/
int ft[maxn];
void update(int p, int x) {
for(; p < maxn; p += p & -p)
ft[p] += x;
}
int sum(int l, int r) {
int ret = 0;
for(--l; l > 0; l -= l & -l) ret -= ft[l];
for(; r > 0; r -= r & -r) ret += ft[r];
return ret;
}
ll n;
vector < ll > v[3];
ll a[maxn] , b[2*maxn];
long long f(ll x){
long long ans = 0;
ll seg [2*n + 5];
for(ll i = 0; i < n; i++){
ll X = v[0][i];
ll Y = v[1][(i+x)%n];
if(Y < X) swap(X,Y);
seg[X] = Y;
seg[Y] = X;
}
ll cnt = 0;
for(ll i = 1; i <= 2*n; i++) ft[i] = 0;
for(ll i = 1; i <= 2*n; i++) b[i] = 0;
for(ll i = 1; i <= 2*n; i++){
ll X = i;
ll Y = seg[i];
if(b[X]){
update(X , 0);
continue;
}
b[Y] = 1;
ll k = sum(X, Y);
ans += k;
update(Y, 1);
}
return ans;
}
void solve(){
cin >> n;
for(ll i = 1; i <= 2*n; i++){
char c;
cin >> c;
if(c == 'B') a[i] = 1;
else a[i] = 0;
v[a[i]].pb(i);
}
long long ans = 0;
ll lo = 0, hg = n-1;
while(lo <= hg){
ll mid = (lo + hg)/2;
long long x1 = f(mid) , x2 = f(mid+1);
if(x1 < x2) ans = max(ans , x2) , lo = mid+1;
else ans = max(ans , x1) , hg = mid-1;
}
cout << ans;
return;
}
int main() {
#ifdef LOCAL
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#else
// freopen("cbarn2.in", "r", stdin);
// freopen("cbarn2.out", "w", stdout);
ios_base::sync_with_stdio(0); cin.tie(NULL); cout.tie(NULL);
#endif
//cout << fixed << setprecision(10);
// work(maxn);
// mem(b,0);
ll T;
T=1;
//cin >> T;
//scanf("%lld",&T);
// ll CT = 0;
//work();
while(T--){
//cout << "Case " << ++CT <<": " ;
//printf("Case %lld: ",++CT);
solve();
}
return 0;
}
