#include<iostream.h>
#include<fstream.h>
int main()
{
	ifstream f("litere.in");
	ofstream g("litere.out");
	int n,i,aux,j,nrinv=0,ind;
	char s[10001];
	int v[26]={};
	f>>n;
	f>>s;
	/*do
	{
		gata=1;
	for(i=0; i<n-1; i++)
	if(s[i]>s[i+1])
	{
		aux=s[i];
		s[i]=s[i+1];
		s[i+1]=aux;
		gata=0;
		nrinv++;
	}
	}
		while(!gata);*/
	for(i=0;i<n;i++)
	{
		aux=s[i];
		ind=(int)s[i]-97;
	
		v[ind]++;
			for(j=ind+1;j<=25;j++)
				if(v[j]>0) nrinv=nrinv+v[j];
	}
	
	g<<nrinv;
	return 0;
}