#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int n,f[305],sol;
int main()
{
    char s;
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>s;
        f[s]++;
        for(int j=s+1;j<='z';j++)
        {
            sol+=f[j];
        }
    }
    fout<<sol;
    return 0;
}