#include <string>
#include <cstring>
#include <cstdio>
#include <iostream>
using namespace std;

#define MAXN (1 << 14)

int N, freq[26];
string A;

int main(void)
{
    freopen("litere.in", "rt", stdin);
    freopen("litere.out", "wt", stdout);

    int res = 0, i, j;

    cin >> N;
    cin >> A;
    
    for(i = 0; i < N; freq[A[i]-'a']++, i++)
     for(j = A[i]-'a'+1; j < 26; j++)
        res += freq[j];

    cout << res << '\n';

    return 0;
}