#include<stdio.h>
int v[100];
int main()
{
int i,n,j,num=0;
char ch;
freopen("litere.in","r",stdin);
freopen("litere.out","w",stdout);
scanf("%d%c",&n,&ch);
for(i=1;i<=n;++i)
{
scanf("%c",&ch);
ch-=96;
v[ch]++;
for(j=ch+1;j<=30;j++)
num+=v[j];
}
printf("%d",num);
fcloseall();
return 0;
}