#include <cstdio>

char a[10100];
bool maimare[128][128];

int main(){
	freopen("litere.in", "r", stdin);
	freopen("litere.out", "w", stdout);
	unsigned n, sol = 0;
	scanf("%d\n", &n);
	gets(a);
	for (unsigned i=n; i<sizeof(a); i++)
		a[i] = 'z'+1;
	for (char c1=0; c1<='z'; c1++)
		for (char c2=0; c2<c1; c2++)
			maimare[c1][c2] = true;
	for (unsigned i=0; i<n; i++)
		for (unsigned j=i+1; j<10000; j++)
			if (maimare[a[i]][a[j]]) sol++;
	printf("%d\n", sol);
}