#include<fstream>
#include<iostream>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int frecv[26];
int main()
{
    int sol=0,n,x,i;
    char s[10001],j;
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>s[i];
         frecv[s[i]-'a']++;
          for(j=s[i]+1;j<='z';j++)
           sol+=frecv[j-'a'];
    }
    g<<sol;
return 0;
}