#include<iostream>
#include<fstream>

char aux;
using namespace std;

int n,ss,p;
char a[100001];
int b[500];

void contor()
{
    int i,j,k;

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;
           // cout<<p<<" "<<ss<<" ";
           }
   // cout<<"\n";
    }

}


int main()
{
    ifstream f ("litere.in");
    ofstream o ("litere.out");
    f>>n;
    f.get();
    f.get(a,n+1);
 contor();
int i;
//for(i=0;i<n;i++)
  //  cout<<a[i]<<" ";
o<<ss;



return 0;}