#include<stdio.h>
FILE *fin,*fout;
int i,v[30],nr,n,t,j,max;
char ch;
int main(){
	fin=fopen("litere.in","r");
	fout=fopen("litere.out","w");
	fscanf(fin,"%d\n",&n);
	for(i=1;i<=n;i++){
		fscanf(fin,"%c",&ch);
		t=ch-'a'+1;
		v[t]++;
		if(t>max){max=t;}
		else{
			for(j=t+1;j<=max;j++){
				nr+=v[j];
			}
		}
	}
	fprintf(fout,"%d",nr);
	fclose(fin);
	fclose(fout);
return 0;}