#include <cstdio>

using namespace std;

const int alph='z'-'a'+1;

int n,o[30],nr;
char c[70000];
int main()
{freopen("litere.in","r",stdin);
 freopen("litere.out","w",stdout);

    scanf("%d",&n);

    int i,j;
    scanf("%c",&c[0]);
        for(i=1;i<=n;++i)
            scanf("%c",&c[i]);

        for(i=1;i<=n;++i)
                {for(j=c[i]-'a'+2;j<=alph;++j)
                    nr+=o[j];
                  o[c[i]-'a'+1]++;
                }

    printf("%d\n",nr);

    fclose(stdin);
    fclose(stdout);


    return 0;
}