#include<stdio.h>
#include<string.h>
FILE *f,*g;
char s[10000];
int i,j,nr,n;

int main()
{
f=fopen("litere.in","r");
g=fopen("litere.out","w");
fscanf(f,"%d\n",&n);
fscanf(f,"%s",s);
for(i=0;i<n-1;i++)
for(j=i+1;j<n;j++)
if(s[i]>s[j])
nr++;
fprintf(g,"%d",nr);
fcloseall();
return 0;
}