#include <cstdio>
using namespace std;
char v[100002];
int f[27];
int main()
{
    FILE *in,*out;
    in=fopen("litere.in","r");
    out=fopen("litere.out","w");
    int l,i,j,op=0;
    fscanf(in,"%d\n",&l);
    fgets(v,100002,in);
    for(i=0;i<l;++i)
    {
        for(j=v[i]-'a'+1;j<27;++j)op+=f[j];
        ++f[v[i]-'a'];
    }
    fprintf(out,"%d\n",op);
    fclose(in);fclose(out);
    return 0;
}