#include <fstream>

using namespace std;

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

int f[30];

int main()
{
    int n,k=0;
    char ch;
    cin>>n;
    for(int j=1;j<=n;j++)
    {
        cin>>ch;
        for(int i=ch-'a'+2;i<='z'-'a'+1;i++)
            k=k+f[i];
        f[ch-'a'+1]++;
    }
    cout<<k;
    return 0;
}