#include <bits/stdc++.h>
using namespace std;
using ll = long long;
struct Fenwick {
int n;
vector<int> t;
Fenwick (int n) : n(n), t(n) {}
void inc(int i, int d=1) {
for (; i < n; i |= i+1)
t[i] += d;
}
int sum(int r) {
int s = 0;
for (; r >= 0; r = (r&(r+1))-1)
s += t[r];
return s;
}
};
signed main() {
cin.tie(0)->sync_with_stdio(0);
int n;
string s;
cin >> n >> s;
vector<vector<int>> ix(128);
vector cnt(128, 0);
for (char c : s) cnt[c]++;
for (int i = 0; i < n*2; i++)
ix[s[i]].push_back(i);
vector left(n*2, 0);
for (char c = 'a'; c <= 'z'; c++) {
assert((int)ix[c].size() == cnt[c]);
for (int i = 0; i < cnt[c]/2; i++) {
left[ix[c][i]] = 1;
}
}
vector<vector<int>> renum(128);
vector arr(n*2, 0);
for (int i = 0, k = 0; i < n * 2; i++) {
if (left[i]) {
arr[i] = k;
renum[s[i]].push_back(k);
k++;
}
}
for (auto& i : renum) reverse(i.begin(), i.end());
for (int i = 0; i < n * 2; i++) {
if (!left[i]) {
arr[i] = n + renum[s[i]].back();
renum[s[i]].pop_back();
}
}
Fenwick tree(n*2);
ll res = 0;
for (int i : arr) {
res += tree.sum(n*2-1) - tree.sum(i);
tree.inc(i);
}
cout << res << endl;
return 0;
}
