#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int f[26];
int main()
{
    int n,x,rasp=0;
    char c;
    fin>>n;
    for(int i=0;i<n;i++)
    {
        fin>>c;
        x=c-97;
        for(int j=x+1;j<26;j++)
        {
            rasp+=f[j];
        }
        f[x]++;
    }
    fout<<rasp;
}