#include <cstdio>
FILE *f,*g;
char a[40100];
int b[30];
int n,i,j,ax,sol;

int main() {
f=fopen("litere.in","r");
g=fopen("litere.out","w");

fscanf(f,"%d",&n);
fscanf(f,"%s",a);
fscanf(f,"%s",a);

for (i=n-1;i>=0;i--) {
	ax=0;
	b[a[i]-'a']++;
	for (j=a[i]-'a'-1;j>=0;j--)
		ax+=b[j];
	sol+=ax;
	}
fprintf(g,"%d",sol);

fclose(g);
return 0;
}