#include <stdio.h>   
int n,i=0,j,nr=0;   
char v[10000];   
int main()   
{freopen("litere.in","rt",stdin);   
scanf("%d\n", &n);   
gets(v);   
while (i<n)   
      {j=i+1;   
      while (j<n)   
       {if (v[i]>v[j]) ++nr;   
       ++j;   
      }   
     ++i;}   
freopen("litere.out","wt",stdout);   
printf("%d\n",nr);   
return 0;}