#include <fstream>
#include <cstring>

using namespace std;

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

int f[150];
int main() {
    
    int n, cnt = 0;
    char s[10002];
    cin >> n;
    cin >> s;
    for(int i = 0; i < n; ++i){
        f[s[i]]++;
        for(int j = s[i] + 1; j <= 'z'; ++j)
            cnt += f[j];
    }
    cout << cnt;
    return 0;
}