#include<stdio.h>
int v[10000];
int main()
{
int n,i;
long num=0;
char ch;
freopen("litere.in","r",stdin);
freopen("litere.out","w",stdout);
scanf("%d",&n);
scanf("%c",&ch);
for(i=1;i<=n;++i)
{
scanf("%c",&ch);
v[i]=ch-'a';
}
int sortat,aux;
do
{
sortat=1;
for(i=1;i<n;++i)
if(v[i]>v[i+1])
{
++num;
aux=v[i];
v[i]=v[i+1];
v[i+1]=aux;
sortat=0;
}
}
while(sortat==0);
printf("%ld",num);
fcloseall();
return 0;
}