#include <stdio.h>
int fr[27];
int main(){
    int n,i,s,j;
    char c;
    FILE *fin,*fout;
    fin=fopen("litere.in","r");
    fout=fopen("litere.out","w");
    fscanf(fin,"%d ",&n);
    s=0;
    for(i=0;i<n;i++){
        fscanf(fin,"%c",&c);
        fr[c-'a'+1]++;
        for(j=c-'a'+2;j<='z'-'a'+1;j++){
            s+=fr[j];
        }
    }
    fprintf(fout,"%d",s);
    return 0;
}