#include <bits/stdc++.h>
using namespace std;
int vf[30];

int main(void){
    ofstream cout("litere.out");
    ifstream cin("litere.in");
    int n;
    cin >> n;
    string s;
    cin >> s;
    long long int ans = 0;
    for(int i = 0;i<s.size();i++){
        vf[s[i]-'a']++;
        for(int j = (s[i]-'a')+1;j<='z'-'a';j++){
            ans += vf[j];
        }
    }
    cout << ans;
}