//
#include <stdio.h>
#include <string.h>

#define in "litere.in"
#define out "litere.out"
#define NMAX 10001

char s[NMAX];
int n, nrsol;

void BubbleSort( char s[], int size );

int main()
{
    FILE *fin = fopen( in, "r" );
    FILE *fout = fopen( out, "w" );

    int i, j;
    char c;
    fscanf( fin, "%d%c", &n, &c );
    fscanf(fin, "%s", &s );
    
    BubbleSort( s, n );
    //fprintf( fout, "%s\n", s );
    /*for ( i = 1; i <= n; ++i  )
    {
        fprintf( fout, "%d ", a[i] );
    } */       
    
    fprintf( fout, "%d\n", nrsol );
    return 0;
}

void BubbleSort( char s[], int size)
{
     int i, j; 
     char temp;
     for ( i = size-1; i >= 0; --i )
     {
         for ( j = 1; j <= i; ++j )
         {
             if ( s[j-1] > s[j] )
             {
                  temp = s[j-1];
                  s[j-1] = s[j];
                  s[j] = temp;
                  nrsol++;
             }
         }
     }
}