# include <iostream>
# include <fstream>
# include <string.h>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
char s[10001];
int main()
{
    int i, n, nr=0, j;
    f>>n;
    for (i=0; i<n; i++)
        f>>s[i];
    for (i=0; i<n-1; i++)
    {
        char minim=s[i];
        int poz=i;
        for (j=i+1; j<n && minim!=s[i-1]+1; j++)
        {
            if (s[j]<minim)
                {minim=s[j];
                poz=j;}
        }
        if (poz!=i)
        {
            for(j=poz; j>i; j--)
            {
                char aux=s[j];
                s[j]=s[j-1];
                s[j-1]=aux;
                nr++;
            }
        }


    }
    g<<nr;
}