#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MX = 2e5 + 5;
int N;
string S;
int curCnt[26], totCnt[26], pos[26][MX];
vector<int> a;
struct fw {
int t[MX];
void upd(int x, int val) {
for(int i = x; i <= N; i += i & -i) t[i] += val;
}
int que(int x) {
int res = 0;
for(int i = x; i > 0; i -= i & -i) res += t[i];
return res;
}
int que(int l, int r) {
return que(r) - que(l - 1);
}
void reset() {
for(int i = 0; i < MX; i++) t[i] = 0;
}
} ft;
int main() {
cin.tie(0); ios_base::sync_with_stdio(0);
cin >> N >> S;
for(auto x : S) a.push_back(x - 'a');
for(auto x : a)
totCnt[x]++;
int i = 0, r = 0;
vector<pair<int,int>> v;
ll ans = 0;
for(auto x : a) {
if(curCnt[x] < totCnt[x] / 2) {
pos[x][curCnt[x]] = i;
ans += r;
} else {
pos[x][curCnt[x]] = i;
v.push_back({pos[x][curCnt[x] - totCnt[x] / 2], r});
r++;
}
curCnt[x]++;
i++;
}
sort(v.begin(), v.end());
ft.reset();
for(auto [l, r] : v) {
ans += ft.que(r + 1, N);
ft.upd(r + 1, 1);
}
cout << ans << '\n';
}
