#include<iostream>
#include<fstream>

char aux;
using namespace std;

int n,ss=0;
char a[10001];


void contor()
{
    int i,j;

for(j=n-1;j>=0;j--)
for(i=j-1;i>=0;i--)
{
    if(a[j]<a[i])
        {
              ss++;
        }


}

}

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;}