#include<fstream>
#include<string.h>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int bifat[30],nr,j,i,a;
char v[10005];
int main()
{
	f>>nr;nr=0;f.get();
	f.getline(v,10005);
	a=strlen(v);
	for(i=0;i<a;i++)
	{
		bifat[v[i]-'a'+1]=1;
		for(j=v[i]-'a'+2;j<='z'-'a'+1;j++)
		{
			if(bifat[j]==1)
			{
				nr++;
			}
		}
	}
	g<<nr;
}