String Reversal

#include<bits/stdc++.h>
using namespace std;

typedef long double ld;
typedef long long int ll;
typedef pair<int, int> pi;
typedef pair<long long, long long> pll;

#define endl '\n'
#define ff first
#define ss second
#define pb push_back
#define int long long
#define sz(v) (int)v.size()
#define inf 2147483647
#define llinf 9223372036854775807
#define all(v) v.begin(),v.end()
#define bp(n) __builtin_popcountll(n)
#define f(i,l,r) for(long long i=l;i<=r;i++)
#define rf(i,r,l) for(long long i=r;i>=l;i--)
#define fast ios_base::sync_with_stdio(false),cin.tie(NULL),cout.tie(NULL)

template<typename T> ostream& operator<<(ostream &os, const vector<T> &v) { os << '{'; string sep; for (const auto &x : v) os << sep << x, sep = ", "; return os << '}'; }
template<typename T, size_t size> ostream& operator<<(ostream &os, const array<T, size> &arr) { os << '{'; string sep; for (const auto &x : arr) os << sep << x, sep = ", "; return os << '}'; }
template<typename A, typename B> ostream& operator<<(ostream &os, const pair<A, B> &p) { return os << '(' << p.first << ", " << p.second << ')'; }

void dbg_out() { cerr << endl; }
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cerr << ' ' << H; dbg_out(T...); }


const int N = 1e5 + 5, mod = 1e9 + 7, bit = 61;


template<typename T>
class fenwick
{
public:
    vector<T> fen;
    int n;

    fenwick() {}

    fenwick(int _n, T val = T())
    {
        n = _n;
        fen = vector<T> (n + 1, val);
    }

    T merge(T a, T b)
    {
        return a + b;
    }

    void update(int idx, T val)
    {
        while (idx <= n)
        {
            fen[idx] = merge(fen[idx], val);
            idx += (idx & -idx);
        }
    }

    T query(int idx)
    {
        T ans{};
        while (idx > 0)
        {
            ans = merge(ans, fen[idx]);
            idx -= (idx & -idx);
        }
        return ans;
    }

    T from(int l, int r)
    {
        return query(r) - query(l - 1);
    }
};

vector<int> adj[26];

signed main()
{
    fast;


#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif

    int t = 1;
    //cin >> t;
    f(tc, 1, t)
    {
        int n;
        cin >> n;
        string fw, bw;
        cin >> fw;
        reverse(all(fw));
        bw = fw;
        reverse(all(fw));
        fw = '#' + fw;
        bw = '#' + bw;
        fenwick<int> obj(n);
        f(i, 1, n)
        {
            int id = fw[i] - 'a';
            adj[id].pb(i);
        }
        int ans = 0;
        rf(i, n, 1)
        {
            int id = bw[i] - 'a';
            int x = adj[id].back();
            adj[id].pop_back();
            int nx = x + obj.query(x);
            obj.update(x, -1);
            ans += i - nx;
        }
        cout << ans << endl;
    }
    return 0;
}