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