#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int v[66004],v1[66004],a[100004],n,x,y,n1,s;
char c[10004];
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>c[i];
        for(int j=c[i]+1;j<='z';j++)
            if(v[j]!=0)
                s+=v[j];
        v[c[i]]++;
    }
    fout<<s;
    return 0;
}