#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pii> vpi;
typedef vector<pll> vpl;
#define pb push_back
#define popb pop_back
#define all(x) (x).begin(), (x).end()
#define ff first
#define ss second
const int nax = 4e5 + 4;
int n;
int col[nax], dp[nax];
vi perm, ot;
int k = 1;
vi st;
void update(int i, int delt)
{
i += k;
st[i] += delt;
i /= 2;
while(i)
{
st[i] = st[2 * i ] + st[2 * i + 1];
i /= 2;
}
}
int query(int p ,int l, int r, int i, int j)
{
if( i > j )
return 0;
if(l >= i && r <= j)
return st[p];
int m = (l + r)/2;
return query(2 * p, l, m ,i ,min(j, m))
+ query(2 * p + 1, m + 1, r, max(i,m + 1), j);
}
int compute(int x)
{
if(x >= n)
return 0 ;
if(dp[x] != -1)
return dp[x];
vpi inter;
int rep = 0;
for(int i= 0 ; i < n; i++)
{
int a = perm[(i + x)%n];
int b = ot[i];
inter.pb({min(a,b), max(a,b)});
}
sort(all(inter));
reverse(all(inter));
for(int i= 0 ; i < n; i++)
{
int a = inter[i].ff;
int b= inter[i].ss;
rep += query(1, 0, k - 1, a, b);
update(a, 1);
update(b, -1);
}
for(int i = 0 ; i < n; i++)
{
int a = inter[i].ff;
int b = inter[i].ss;
update(a, -1);
update(b, 1);
}
//cout << x << " " << rep <<"\n";
return dp[x] = rep;
}
int32_t main()
{
ios::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
memset(dp, -1,sizeof(dp));
cin >> n;
k = 1;
while(k < 2 * n)
k = (k << 1);
st.assign(2 * k + 1, 0 );
for(int i = 0 ; i < 2 * n ; i++)
{
char c; cin >> c;
if(c == 'B')
col[i] = 1;
if(col[i])
perm.pb(i);
else
ot.pb(i);
}
int ans = 0 ;
int debut = 0;
int fin = n - 1;
int rep = debut;
while(debut <= fin )
{
int mid = (debut + fin)/2;
//cout << mid << ' ' << compute(mid) << ' ' << compute(mid + 1) << "\n";
if(compute(mid + 1) > compute(mid))
{
rep = mid + 1;
debut = mid + 1;
ans = max(ans, compute(mid + 1));
}
else
{
rep = mid ;
fin = mid - 1;
ans = max(ans, compute(mid ));
}
}
cout << ans;
}
