#include <cstdio>
#include <cstring>
using namespace std;
int a[31];
int n,m,i,j,s;
char c;
int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%c",&c);
        m=c-'a'+1;
        for(j=m+1;j<='z'-'a'+1;j++)
            s+=a[j];
        a[m]++;
    }
    printf("%d\n",s);
    return 0;
}