#include<bits/stdc++.h>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
using namespace std;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int>vi;
typedef vector<vector<int>>vvi;
typedef vector<ll>vl;
typedef vector<vl> vvl;
typedef pair<int,int>pi;
typedef pair<ll,ll> pl;
typedef vector<pl> vpl;
typedef vector<ld> vld;
typedef pair<ld,ld> pld;
typedef vector<pi> vpi;
//typedef tree<ll, null_type, less_equal<ll>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
template<typename T> ostream& operator<<(ostream& os, vector<T>& a){os<<"[";for(int i=0; i<ll(a.size()); i++){os << a[i] << ((i!=ll(a.size()-1)?" ":""));}os << "]\n"; return os;}
#define all(x) x.begin(),x.end()
#define YES out("YES")
#define NO out("NO")
#define out(x){cout << x << "\n"; return;}
#define GLHF ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define print(x){for(auto ait:x) cout << ait << " "; cout << "\n";}
#define pb push_back
#define umap unordered_map
template<typename T1, typename T2> istream& operator>>(istream& is, pair<T1, T2>& p){is >> p.first >> p.second;return is;}
template<typename T1, typename T2> ostream& operator<<(ostream& os, pair<T1, T2>& p){os <<"" << p.first << " " << p.second << ""; return os;}
void usaco(string taskname){
string fin = taskname + ".in";
string fout = taskname + ".out";
const char* FIN = fin.c_str();
const char* FOUT = fout.c_str();
freopen(FIN, "r", stdin);
freopen(FOUT, "w", stdout);
}
template<typename T>
void read(vector<T>& v){
int n=v.size();
for(int i=0; i<n; i++)
cin >> v[i];
}
template<typename T>
vector<T>UNQ(vector<T>a){
vector<T>ans;
for(T t:a)
if(ans.empty() || t!=ans.back())
ans.push_back(t);
return ans;
}
ll ceil(ll a,ll b){
ll ans=a/b;
if(a%b)ans++;
return ans;
}
ld log (ld a,ld b){return log(b)/log(a);}
ll power(ll base, ll exp,ll M=1e18){//(base^exp)%M
ll ans=1;
while(exp){
if(exp%2==1)ans=((ans%M)*(base%M))%M;
base=((base%M)*(base%M))%M;
exp/=2;
}
return ans;
}
string to_base(int n,int new_base){
string s;
int nn=n;
while(nn){
s+=to_string(nn%new_base);
nn/=new_base;
}
reverse(all(s));
return s;
}
ll gcd(ll a,ll b){
if(a<b)swap(a,b);
if(b==0)return a;
return gcd(b,a%b);
}
ll lcm(ll a,ll b){
ll x= (a/gcd(a,b));
return b*x;
}
vl generate_array(ll n,ll mn=1,ll mx=1e18+1){
if(mx==1e18+1)
mx=n;
vl ans(n);
for(int i=0; i<n;i++)
ans[i]=(mn+rand()%(mx-mn+1));
return ans;
}
string substr(string s,int l,int r){
string ans;
for(int i=l; i<=r; i++)
ans+=s[i];
return ans;
}
void solve();
int main(){
GLHF;
int t=1;
//cin >> t;
while(t--)
solve();
}
struct seg{
vl t;
ll sz=1;
seg(int n){
while(sz<=2*n)
sz*=2;
t.resize(sz);
}
void upd(int i,int v){
t[i+=sz/2]+=v;
while(i){
t[i/2]=t[i]+t[i^1];
i/=2;
}
}
ll qur(ll l,ll r){
ll ans=0;
for(l+=sz/2,r+=sz/2 +1; l<r; l/=2,r/=2){
if(l&1)ans+=t[l++];
if(r&1)ans+=t[--r];
}
return ans;
}
};
ll count(string s,string t){
int n=s.size();
vi to(n);
vvi c(26);
for(int i=0; i<n; i++)
c[s[i]-'a'].pb(i);
for(int i=n-1; 0<=i; i--){
to[c[t[i]-'a'].back()]=i;
c[t[i]-'a'].pop_back();
}
ll ans=0;
seg ST(n);
for(int i=0; i<n; i++){
ans+=ST.qur(to[i]+1,n);
ST.upd(to[i],1);
}
return ans;
}
void solve() {
int n;
cin >> n;
string s;
cin >> s;
string s1,s2;
map<int,int>cnt,cur;
for(char c:s)
cnt[c]++;
for(int i=0; i<2*n; i++){
cur[s[i]]++;
if(2*cur[s[i]]<=cnt[s[i]])
s1+=s[i];
}
cur.clear();
for(int i=2*n-1; 0<=i ;i--){
cur[s[i]]++;
if(2*cur[s[i]]<=cnt[s[i]])
s2+=s[i];
}
reverse(all(s2));
out(min(count(s,s1+s1),count(s,s2+s2)))
}
