#include <iostream>
#include <fstream>
using namespace std;
ifstream in("litere.in");
ofstream out("litere.out");
char T[10005];
int f[30];
int main()
{
    int n;
    in>>n;
    in.getline(T,0);
    in.getline(T,10000);
    int v[n+1];
    for(int i=0;i<n;i++)
        v[i+1]=(int)(T[i]-'a');
    int nr=0;
    for(int i=1;i<=n;i++)
    {
        int cnt=0;
        for(int j=v[i]+1;j<= 'z'-'a';j++)
            cnt=cnt+f[j];
        nr=nr+cnt;
        f[v[i]]++;
    }
    out<<nr;
    return 0;
}