#include <fstream>
#include <cstring>
#include <algorithm>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
int f[160];
char s[10020];
int n,i,j,cnt;
int main() {
    cin>>n;
    cin>>s;
    for (i=0;i<n;i++) {
        f[s[i]]++;
        for (j=s[i]+1;j<=122;j++) {
            cnt+=f[j];
        }
    }
    cout<<cnt;
}