#include <cstdio>
#define FIN "litere.in"
#define FOUT "litere.out"
#define N 10010
int n,a[27],rez,r[27];
char s[N];
void read()
{
	int i;
	freopen(FIN, "r", stdin);
	scanf("%d\n", &n);
	fgets(s,n + 1,stdin);
	for (i = 0; i < n; ++i)
		++a[s[i] - 'a'];
}
/*
void solve()
{
	int i,j;
	for (i = 0; i < 26; ++i)
	{
		rez += r[i];
		for (j = 0; j < i; ++j)
			++r[j];
	}
}
*/
void solve()
{
	int i,j;
	for(i = 0 ; i < n ; ++i)
	{
		rez += r[s[i] - 'a'];
		for(j = 0; j < s[i] - 'a'; ++j)
			++r[j];
	}
}
void write()
{
	freopen(FOUT,"w",stdout);
	printf("%d\n", rez);
}
int main()
{
	read();
	solve();
	write();
}