#include <fstream>
#include <cstring>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
int n,fr[30],k,i,j,nr;
char c[10005];
int main()
{
    cin>>n;
    for(i=1;i<=n;i++)
        cin>>c[i];
    for(i=1;i<=n;i++)
    {
        for(j=c[i]-'a'+2;j<=26;j++)
        {
            nr=nr+fr[j];
        }
        fr[c[i]-'a'+1]++;
    }
    cout<<nr;
    return 0;
}