#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;
}