#include<stdio.h>
#define LMAX 10001

char s[LMAX];  int n,alpha[26],i,nrinv;

void inv(int poz)
{alpha[poz]++;
 for(int j=poz+1;j<=25;j++)
 if(alpha[j]>=1) nrinv+=alpha[j];  }

main()
{freopen("litere.in","r",stdin);
 freopen("litere.out","w",stdout);

 scanf("%d",&n);  scanf("%c",&s[0]);

 for(i=0;i<n;i++) scanf("%c",&s[i]);

 for(i=0;i<n;i++)
 inv(s[i]-97);

 printf("%d\n",nrinv);}