#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
char s[10005], j;
int n, x, y, i, v[130];
int main()
{
    fin>>n;
    fin>>s;
    x=0;
    v[s[0]-'a']=1;
    for (i=1; i<n; i++)
    {
        y=x;
        v[s[i]-'a']++;
        for (j=s[i]-'a'+1; j<='z'-'a'; j++)
            y=y+v[j];
        x=y;
    }
    fout <<y << endl;
    return 0;
}