#include <stdio.h>
int frecv[26];

int main()
{
    FILE *in,*out;
    in=fopen("litere.in","r");
    out=fopen("litere.out","w");
    int n,i,rez=0,j;
    char a;
    fscanf(in,"%d ",&n);
    for(i=0;i<n;i++)
    {
        a=fgetc(in);
        frecv[a-'a']++;
        a-='a';
        for(j=a+1;j<26;j++)
        {
            rez+=frecv[j];
        }
    }
    fprintf(out,"%d",rez);
    return 0;
}