#include <bits/stdc++.h>
#define DAU  ios::sync_with_stdio(false); fin.tie(0); fout.tie(0);
#define PLEC fin.close(); fout.close(); return 0;
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
char s[10001];
int n, res, freq[26];
int main()
{
    DAU
    fin >> n >> (s + 1);
    for (int i = 1; i <= n; ++i)
    {
        for (int v = s[i] - 'a' + 1; v < 26; ++v)
            res += freq[v];
        ++freq[s[i] - 'a'];
    }
    fout << res;
    PLEC
}