XMAX

#include<bits/stdc++.h>
#define pll pair<ll, ll>
#define fi first
#define se second
#define pb push_back
#define task "XMAX"
#define pii pair<ll, pll>
using namespace std;
using ll = long long;
const int  N = 2e5+5;
const ll mod = 1e9+7;
const ll base = 350;
const ll cs = 331;
ll m, n, t, k, tong, q, ans, a[N], b[N], c[N], fe[N];
vector<ll> adj[N], kq;
string s;
void add(ll id)
{
    for(; id <= n; id += id & -id)++fe[id];
}
ll get(ll id)
{
    ll total = 0;
    for(; id; id -= id & -id)total += fe[id];
    return total;
}
ll pw(ll k, ll n)
{
    ll total = 1;
    for(; n; n >>= 1)
    {
        if(n & 1)total = total * k % mod;
        k = k * k % mod;
    }
    return total;
}
bool cmp(const ll& x, const ll& y)
{
    return a[x] < a[y];
}
void sol()
{

    cin >> n >> s;
    for(int i = 0; i < n; i ++)
    {
        k = s[i] - 'a';
        adj[k].pb(i+1);
    }
    for(int i = 0; i < 26; i ++)reverse(adj[i].begin(), adj[i].end());
    reverse(s.begin(), s.end());
    for(int i = 0; i < n; i ++)
    {
        int j = s[i] - 'a';
        k = adj[j].back();
        //cout << k <<" ";
        adj[j].pop_back();
        ans += get(n) - get(k);
        add(k);
    }
    cout << ans;
}
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    if(fopen(task".inp", "r"))
    {
        freopen(task".inp", "r", stdin);
        freopen(task".out", "w", stdout);
    }
    int ntest = 1;
    //cin >> ntest;
    while(ntest -- > 0)
    sol();
}