#include <fstream>
#define DIM 10005
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int n,i,j,cnt,f[26];
char s[DIM];
int main() {
    fin>>n>>s;
    for (i=0;i<n;i++) {
        f[s[i]-'a']++;
        for (j=s[i]+1;j<='z';j++)
            cnt+=f[j-'a'];
    }
    fout<<cnt;
    return 0;
}
/**
a b a k l j f d o q  p  a  w  k  f  j  a
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

a a  a  a b d  f f  j j  k k l o  p q  w
0 2 11 16 1 7 14 6 15 5 13 3 4 8 10 9 12
**/