#include <stdio.h>
int n,v[10001],i,j,nr,t;
char c;
int main() {
	FILE *f=fopen("litere.in","r");
	FILE *g=fopen("litere.out","w");
	fscanf(f,"%d\n",&n);
	for(i=1;i<=n;i++) {
		fscanf(f,"%c",&c);
		v[i]=c-'a';
	}
	for(i=1;i<=n;i++)
		for(j=i+1;j<=n;j++)
			if(v[i]>v[j]) {
				nr++;
				t=v[i];
				v[i]=v[j];
				v[j]=t;
			}
	fprintf(g,"%d",&nr);
}