#include <stdio.h>

#define NMAX 10000
#define FIN "litere.in"
#define FOUT "litere.out"

int n,i,j,nr=0;
char v[NMAX];

int main()
{
freopen(FIN,"rt",stdin);
freopen(FOUT,"wt",stdout);

scanf("%d\n", &n);
gets(v);
i=0;
while (i<n)
      {
      j=i+1;
      while (j<n)
	   {
      if (v[i]>v[j]) nr++;
      j++;
      }
     i++;
     }
printf("%d\n",nr);
return 0;
}