#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin ("litere.in");
ofstream cout("litere.out");
int n,i,j,nr,rez,poze;
int f[260];
char s[10005];
int main () {
    cin>>n;
    cin>>s;
    for (i=0;i<n;i++) {
        f[s[i]]++;
        for (j=s[i]+1;j<=122;j++) {
            rez+=f[j];
        }
    }
    cout<<rez;
}