#include <cstdio>
#include <cstdlib>
#include <cstring>
using namespace std;


char s[10010], c;
int sol, v[30], N;

int main()
{
    freopen("litere.in", "r", stdin);
    freopen("litere.out", "w", stdout);
    int i, j;
    scanf("%i\n", &N);
    fgets(s, 10010, stdin); 
    for(i = 0; i < N; i++)
    {
          c = s[i] - 'a' + 1;
          for(j = c + 1; j < 27; j++) sol += v[j];
          v[c] ++;
    }
    printf("%i\n", sol);
    return 0;
}