#include <fstream>
#define nmax 1005

using namespace std;

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

int v[nmax];

int n;

long long rasp;

char a ,b;

int main()
{
    in>>n;
    for(int i=1; i<=n; i++)
    {
        in>>a;
        for( b = a+1; b <= 'z'; b++)
        {
            rasp+= v[b];
        }
        v[a]++;
    }
    out<<rasp;
    return 0;
}