#include <bits/stdc++.h>
#ifndef ONLINE_JUDGE
#include "lib.h"
#endif
#define rep(i, min, max) for(int i = (min); i <= (max); ++i)
#define nrep(i, max, min) for(int i = (max); i >= (min); --i)
#define reads(str) (scanf("%s", str + 1), strlen(str + 1))
#define case() int Ts = read(); rep(T, 1, Ts)
#define putf(flag) puts((flag) ? "YES" : "NO")
#define put(x) printf("%d ", x)
#define putl(x) printf("%lld ", x)
#define endl() putchar('\n')
using namespace std;

typedef long long ll;
inline int read()
{
    int now=0; bool nev=false; char c=getchar();
    while(c<'0' || c>'9') { if(c=='-') nev=true; c=getchar(); }
    while(c>='0' && c<='9') { now=(now<<1)+(now<<3)+(c&15); c=getchar(); }
    return nev?-now:now;
}

const int N = 1e6 + 10;

char c[N];
ll a[N], s[N];

int main() {
    int n = read(), m = n * 2;
    reads(c);
    rep(i, 1, m) {
        if(c[i] == 'B') ++a[i];
        else --a[i <= n ? i + n : i - n];
    }
    rep(i, 1, m) s[i] = s[i - 1] + a[i];
    sort(s + 1, s + m + 1);
    ll sum = 1ll * n * (n - 1), mid = s[(m + 1) / 2];
    rep(i, 1, m) sum -= abs(s[i] - mid);
    putl(sum / 2);
}