#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");

int n, ans, s[30];
char v[10001];

int main()
{
    fin>>n;
    int i,j;
    for (i=1; i<=n; ++i)
        fin>>v[i];
    for (i=1; i<=n; ++i)
    {
        s[v[i]-96]++;
        for (j=v[i]-96+1; j<=26; ++j)
            ans+=s[j];
    }
    fout<<ans;
    return 0;
}