#include<stdio.h>
int s,l,nr,a,n,i,k;
char w[27]={'0','a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
char v[10002];

int main(){
FILE *f=fopen("litere.in","r");
fscanf(f,"%d\n",&n);
  for(i=1;i<=n;i++){
  fscanf(f,"%c",&v[i]);
  }

fclose(f);

 for(i=1;i<=26;i++){
 nr=0;

   for(k=1;k<=n;k++){
   if(v[k]>w[i]&&v[k]!=0)nr++;
   if(v[k]==w[i]){v[k]=0;s+=nr,nr=0;k=0;}

   }

 }

FILE *g=fopen("litere.out","w");
fprintf(f,"%d",s);
fclose(g);

return 0;
}