#include <bits/stdc++.h>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
unsigned long long answer;
char s[10005];
int n, vf[32];
int main()
{
    int i, j, l;
    bool ok;
    fin>>n;
    fin.get();
    fin.getline(s,sizeof(s));
    l=strlen(s)-1;
    for(i=0; i<=l; i++)
    {
        vf[s[i]-'a']++;
        for(j=s[i]-'a'+1; j<='z'-'a'; j++) answer+=(unsigned long long)vf[j];
    }
    fout<<answer<<'\n';
    return 0;
}