#include<stdio.h>
int f[26];
int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    int n,i,j,s=0;char ch;
    scanf("%d\n",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%c",&ch);
        int litera=ch-'a'+1;
        for(j=litera+1;j<=26;j++)
            s=s+f[j];
        f[litera]++;
    }
    printf("%d\n",s);
    return 0;
}