#include<cstdio>
#define Nmax 150

FILE *f=fopen("litere.in","r");
FILE *g=fopen("litere.out","w");

using namespace std;

int n,v[Nmax];
void read()
{
	int i,contor=0,j;
	char c;
	fscanf(f,"%d",&n);
	fscanf(f,"%c",&c);
	for(i=1;i<=n;i++)
	{
		fscanf(f,"%c",&c);
		v[c]++;
		for(j='a';j<='a'+27;j++)
			if(c<j)
				contor=contor+v[j];
	
	}
	fprintf(g,"%d",contor);
}
int main()
{
	read();
	return 0;
}