#include <stdio.h>
#include <stdlib.h>

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