#include<stdio.h>
#define NMAX 10010

char s [ NMAX ] ;

int main ( )
     {

         freopen ( "litere.in" , "r" , stdin ) ;
         freopen ( "litere.out" , "w"  , stdout ) ;

         int n , i ;

         scanf ( "%d " , & n ) ;

         gets ( s ) ;

         //buble sort
         long nr = 0 ;

         int sortat = 0 ;

         while ( ! sortat )
              {

                  sortat = 1 ;
                  for ( i = 0; i < n - 1  ; i ++ )
                     if ( s [ i ] > s [ i + 1 ] )
                       {

                           char aux = s [ i ] ;
                           s [ i ] = s [ i + 1 ] ;
                           s [ i + 1 ] = aux ;
                           nr ++ ;
                           sortat = 0 ;

                       }

              }

         printf ( "%ld" , nr ) ;

         return 0 ;


     }