#include<iostream.h>
#include<stdio.h>
FILE *f=fopen("litere.in","r"),*g=fopen("litere.out","w");
int a[10001];
int main()
{
	int n,i,j,op=0;
	char c[10001];
	int t=1,ul;
	fscanf(f,"%d %s",&n,&c);
        ul=n-1;
	for(i=122;i>=97;i--)
	{
		for(j=0;j<ul;j++)
		{
			if((int)c[j]==i)
			{
				while(c[j]<c[ul])
                                 ul--,op++;
				a[ul]+=ul-j;
				char aux=c[j];
				c[j]=c[ul];
                                c[ul]=aux;
				ul--;
			}
		}
	}
	for(i=0;i<n;i++)
         op+=a[i];
	fprintf(g,"%d\n",op);
	return 0;
}