#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
#define int long long
typedef pair<int,int> pii;
typedef vector<pii> vii;
typedef vector<int> vi;
typedef vector<bool> vb;
#define FOR(i,a) for(int i=0;i<(a);i++)
#define FORE(i,a,b) for(int i=(a);i<(b);i++)
#define all(x) x.begin(),x.end()
#define fi first
#define se second
#define pb push_back
#define sp <<" "<<
#define cont(x) for(auto el:x) cout<<el<<' ';cout<<endl;
#define contp(x) for(auto el:x) cout<<el.fi<<'-'<<el.se<<' ';cout<<endl;
#define DEBUG(x) cout<<#x sp x<<endl;
#define carp(x,y) ((x%MOD)*(y%MOD))%MOD
#define topla(x,y) ((x%MOD)+(y%MOD))%MOD
#define mid (l+r)/2
const int MAXN=5e5+5;
const int INF=1e18;
vi tut[MAXN];
int git[MAXN], val[MAXN];
int t[MAXN*4];
int n;
int query(int nd){
int cev=0;
while(nd>0){
cev+=t[nd];
nd-=nd&-nd;
}
return cev;
}
void update(int nd,int val){
while(nd<=n){
t[nd]+=val;
nd+=nd&-nd;
}
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cin>>n;
string s;
cin>>s;
int len=2*n;
FOR(i,len) tut[s[i]-'a'].pb(i);
vi sol,sag;
//first get each letter to desired half
int bir=0;
FOR(i,30){
if(tut[i].empty()) continue;
int sz=tut[i].size();
FOR(j,sz/2){
git[tut[i][j]]=tut[i][sz/2+j];
sol.pb(tut[i][j]);
sag.pb(tut[i][sz/2+j]);
}
}
sort(all(sol));
sort(all(sag));
FOR(i,n) bir+=sol[i]-i;
vi val(2*n,0);
int say=1;
FOR(i,len){
if(val[i]!=0) continue;
val[i]=val[git[i]]=say;
say++;
}
FOR(i,sag.size()) sag[i]=val[sag[i]];
//find inversions
int iki=0;
FOR(i,sag.size()){
int el=sag[i];
iki+=query(n)-query(el);
update(el,1);
}
cout<<bir+iki<<endl;
}
