#include <cstdio>
#define filein "litere.in"
#define fileout "litere.out"
using namespace std;

int N;
int fv[30];

int main()
{
    FILE *in,*out;
    in=fopen(filein,"r");
    out=fopen(fileout,"w");
    fscanf(in,"%d\n",&N);
    register int i,j;
    char c;
    long long s=0;
    for (i=1; i<=N; i++)
    {
        fscanf(in,"%c",&c);
        fv[c-'a']++;
        for (j=c-'a'+1; j<=26; j++)
            s=s+(long long) fv[j];
    }
    fprintf(out,"%lld",s);
    fclose(in);
    fclose(out);
    return 0;
}