#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define fi first
#define se second
#define ll long long
#define ld long double
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define mpp make_pair
#define ve vector
using namespace std;
using namespace __gnu_pbds;
template<class T> using oset = tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
const ll inf = 1e18; const int iinf = 1e9;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
template <typename T> inline bool chmin(T& a, T b) { return (a > b ? a = b, 1 : 0); }
template <typename T> inline bool chmax(T& a, T b) { return (a < b ? a = b, 1 : 0); }
const int N = 2e5 + 10; int f[N];
inline void upd(int i, int x) { i += 2; for (; i < N; i += i & -i) f[i] += x; }
inline int get(int i) { i += 2; int ans = 0; for (; i; i -= i & -i){ans+=f[i];} return ans; }
inline void solve() {
int n;
string s;
cin >> n >> s; n *= 2;
ve<int> right(n), left(n);
ve<ve<int>> occ(26);
for (int i = 0; i < n; ++i) occ[s[i] - 'a'].pb(i);
for (int c = 0; c < 26; ++c) {
int s = sz(occ[c]);
for (int i = s / 2; i < s; ++i) right[occ[c][i]] = 1;
for (int i = 0; i < s / 2; ++i) left[occ[c][i]] = 1;
}
ve<int> pos(n);
int usl = 0, usr = 0;
for (int i = 0; i < n; ++i) {
if (left[i]) pos[i] = usl++;
else pos[i] = n / 2 + usr++;
}
ll ans = 0;
for (int i = n - 1; ~i; --i) {
ans += get(pos[i] - 1);
upd(pos[i], +1);
}
string res; res.resize(n);
for (int i = 0; i < n; ++i) {
res[pos[i]] = s[i];
}
int si = sz(res);
string a = res.substr(0, si / 2);
string b = res.substr(si / 2, si / 2);
ve<ve<int>> ps(26);
for (int i = sz(a) - 1; ~i; --i) ps[a[i] - 'a'].pb(i);
ve<int> tr(sz(b));
for (int i = 0; i < sz(b); ++i) {
tr[i] = ps[b[i] - 'a'].back();
ps[b[i] - 'a'].pop_back();
}
memset(f, 0, sizeof(f));
for (int i = sz(b) - 1; ~i; --i) {
ans += get(tr[i] - 1);
upd(tr[i], +1);
}
cout << ans;
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int q = 1; // cin >> q;
while (q--) solve();
cerr << fixed << setprecision(3) << "Time execution: " << (double)clock() / CLOCKS_PER_SEC << endl;
}
