#include <stdio.h>
int main(){
	FILE *f,*g;
	long a[10002],i,j,n;
	char ch;
	f=fopen("litere.in","r");
	g=fopen("litere.out","w");
	fscanf(f,"%ld\n",&n);
	for(i=1;i<=n;i++){
		fscanf(f,"%c",&ch);
		a[i]=long(ch-'a');
	}
	long nr=0;
	for(i=1;i<=n-1;i++)
		if(a[i]!=0)
			for(j=i+1;j<=n;j++)
				if(a[i]>a[j]) nr++;
	fprintf(g,"%ld\n",nr);
	fclose(f);
	fclose(g);
	return 0;
}