#include <bits/stdc++.h>

using namespace std;

ifstream in("litere.in");
ofstream out("litere.out");

string s;

int f[27];

int main() {

    int n,rez=0;

    in>>n;

    in.get();

    in>>s;

    for (int i = 0; i < s.size(); ++i) {
        f[s[i]-'a']++;

        for (int j = s[i]-'a'+1; j < 26; ++j) {
            rez+=f[j];
        }
    }

    out<<rez;

    return 0;
}