#include<stdio.h>
long n,cn,i,j,k;
long nr,f[27];
char v[10001];
int main()
{
freopen("litere.in","r",stdin);
freopen("litere.out","w",stdout);
scanf("%ld\n",&n);
for(i=0;i<n;i++)
  {
  scanf("%c",&v[i]);
  f[v[i]-'a']++;
  }

cn=n;
for(i=0;i<n;i++)
  {
  while(f[i])
    {
    for(j=0;j<cn;j++)
      if(v[j]-'a'==i)
        {
        nr=nr+j;
        for(k=j+1;k<cn;k++)
          v[k-1]=v[k];
        break;
        }
    cn--;
    f[i]--;
    }
  }
printf("%ld",nr);
return 0;
}