#include <bits/stdc++.h>
#define MOD 104659
#define ull unsigned long long

using namespace std;

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

int fr[30];

int main()
{
    int n,i,S= 0,j;
    char a;
    fin>>n;
    for(i = 0; i < n; i++){
        fin>>a;
        fr[a-'a']++;
        for(j = a-'a'+1; j <= 'z'-'a'; j++){
            S += fr[j];
        }
    }
    fout<<S<<endl;
    return 0;
}