#include <stdio.h>
int a[26],n,R,i,j;char s[100100];int main(){fscanf(fopen("litere.in","r"),"%d\n%s",&n,s+1);for(i=1;i<=n;++i)for(a[(j=s[i]-'a'+1)-1]++;j<26;++j)R+=a[j];fprintf(fopen("litere.out","w"),"%d",R);return 0;}