#include <stdio.h>
#define NMAX 30010

FILE *f = fopen("litere.in", "r"), *g = fopen("litere.out", "w");

int n, alfabet[30];
char sir[NMAX];
long long suma = 0;


int main()
  {
    fscanf(f, "%d\n", &n);
    fgets(sir, NMAX, f);
    fclose(f);
    
    alfabet[sir[0] - 'a']++;
    for (int i = 1; i < n; i++)
       {for (int j = 'z'; j > sir[i]; j--)
            suma += alfabet[j - 'a'];
        alfabet[sir[i] - 'a']++;
       }

    fprintf(g, "%lld", suma);
    fclose(g);
    return 0; 
  }