#include <fstream>
#include <queue>
#define ll long long

using namespace std;

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

int n;
int f[30];
char s[10001];
ll ans = 0;

int main()
{
    cin >> n >> s;
    for (int i = 0; s[i]; i++)
    {
        f[s[i] - 'a']++;
        for (int j = s[i] + 1; j <= 'z'; j++)
            ans += 1LL * f[j - 'a'];
    }

    cout << ans;

    return 0;
}