#include <stdio.h>
#include<vector>
char v[10000];
int main(){
    FILE *fin=fopen("litere.in","r");
    FILE *fout=fopen("litere.out","w");
    int n,i,j,t=0;

    fscanf(fin,"%d ",&n);
    fscanf(fin,"%s",&v);
    std::vector<int>poz[26];
    for(i=0; i<n; i++){
        for(j=v[i]-'a'+1; j<='z'-'a'; j++)
            t+=poz[j].size();
        poz[v[i]-'a'].push_back(i);
    }
    fprintf(fout,"%d\n",t);
    fclose(fin);
    fclose(fout);
    return 0;
}