#include <stdio.h>

using namespace std;

int w[27],q=0,n;
char s[10001];

void citire()
{
    freopen("litere.in","r",stdin);
    scanf("%d\n", &n);
    fgets(s,10000,stdin);
    fclose(stdin);

}

void litere()
{
    for (int i=0; i<n; i++)
    {
        w[s[i]-96]++;
        for (int j=s[i]-96+1; j<27; j++)
            q+=w[j];
    }
}

int main()
{
    citire();
    litere();
    freopen("litere.out","w",stdout);
    printf("%d",q);
    fclose(stdout);
    return 0;
}