#include <fstream>
using namespace std;

ifstream in("litere.in");
ofstream out("litere.out");

int x[100],i,n,num,j;
char c;

int main()
{
    in>>n;
    in.get(c);
    while(c=='\n')
        in.get(c);
    in.unget();
    for(i=1; i<=n; i++)
    {
        in.get(c);
        for(j=c-'a'+1; j<='z'-'a'; j++)
            num+=x[j];
        x[c-'a'] ++;
    }
    out<<num;
    return 0;
}