#include<stdio.h>

#define N 101

#define U 280

int n,ap[U];

char v[N];

void calcul()
{
	int rez=0;
	for( int i=0 ; i<n ; ++i )
		for( int j=0 ; j<i ; ++j )
			if( v[j] > v[i] )
				++rez;
	printf("%d\n",rez);
}

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d%s",&n,&v);
	calcul();
	return 0;
}