#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
const int Nmax=10005;
int N,S,NR[28];
int A[Nmax];
void Read()
{
    fin>>N;
    for(int i=1;i<=N;++i)
    {
        char x;
        fin>>x;
        A[i]=x-96;

    }
}



void Solve()
{

    for(int i=1;i<=N;++i)
    {
        NR[A[i]]=NR[A[i]]+1;
        for(int j=A[i]+1;j<=27;++j)
        {
          S=S+NR[j];
        }

    }

    fout<<S<<"\n";
}

int main()
{
    Read();

 Solve();

    return 0;
}