#include <fstream>
using namespace std;
     ifstream in("litere.in");
     ofstream out("litere.out");
int main()
{
   int n,k=0,a[10005]; string s ;
   in>>n;
    in>>s;
    for ( int i=0; i<n; i++ )
        a[i]=(int)s[i];
    for ( int i=0; i<n-1; i++ )
    {
        bool ok=1;
        for ( int j=0; j<n-1; j++ )
        if( (int) a[j] > (int)a[j+1] ) { swap(a[j], a[j+1]); k++; ok=0;}
        if( ok ) break;
    }

    out<<k;
    return 0;
}