#include <bits/stdc++.h>
using namespace std;
const int N = 100'000 + 10;
int n;
string s;
int bit[N];
inline void upd(int i, int x) {
for (; i <= n; i += i & -i) bit[i] += x;
}
inline int que(int i) {
int ret = 0;
for (; i; i -= i & -i) ret += bit[i];
return ret;
}
int totalCnt[26];
int pos[26][N];
int32_t main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n;
cin >> s;
s = '@' + s;
for (int i = 1; i <= 2 * n; ++i) totalCnt[s[i] - 'a'] += 1;
vector<int> cnt(26);
{
for (int i = 1, it = 0; i <= 2 * n; ++i) {
int ch = s[i] - 'a';
cnt[ch] += 1;
if (cnt[ch] <= totalCnt[ch] / 2)
pos[ch][cnt[ch]] = ++it;
}
}
long long answer = 0;
for (int i = 2 * n; i >= 1; --i) {
int ch = s[i] - 'a';
if (cnt[ch] <= totalCnt[ch] / 2) upd(1, 1);
else {
int rank = pos[ch][(cnt[ch] - 1) % (totalCnt[ch] / 2) + 1];
answer += que(rank);
upd(rank, 1);
}
cnt[ch] -= 1;
}
cout << answer << "\n";
}
