#include<stdio.h>
FILE *fin=fopen("litere.in","r");
FILE *fout=fopen("litere.out","w");
int n,i,j,fr[256],s;
char ch;

int main(){

    fscanf(fin,"%d\n",&n);

    for(i=1;i<=n;i++){
        fscanf(fin,"%c",&ch);
        fr[ch]++;

        for(j=ch+1;j<='z';j++){
            s+=fr[j];
        }
    }

    fprintf(fout,"%d",s);



return 0;
}