#include <stdio.h>
#include <string.h>

int main()
{
	FILE* fin = fopen("litere.in", "r");
	FILE* fout = fopen("litere.out", "w");

	int N;
	fscanf(fin, "%d", &N);

	char sir[10001];

	fscanf(fin, "%s", sir);

	long long int ct = 0;

	int freq[30];
	memset(freq, 0, 30*sizeof(int));
	int sum[30];
	memset(sum, 0, 30*sizeof(int));
	for (int i = 0; i < N; i++)
	{
		int idx = sir[i] - 'a';
		freq[idx]++;
		for (int j = idx; j >= 0; j--)
			sum[j]++;
		ct += sum[idx+1];
	}

	fprintf(fout, "%lld", ct);

	fclose(fin);
	fclose(fout);

	return 0;
}