#include<iostream>
#include<fstream>


using namespace std;

int n,ss,p;
char a[10001];


void contor()
{
    int i,j;

for(j=97;j<=122;j++)
    {p=0;
        for(i=n-1;i>=0;i--)
           {
              if(a[i]==char(j))
                                p++;

              if(a[i]>char(j))
                            ss=ss+p;

           }

    }

}


int main()
{
    ifstream f ("litere.in");
    ofstream o ("litere.out");
    f>>n;
    f.get();
    f.get(a,n+1);
 contor();
int i;

o<<ss;



return 0;}