#include <iostream>
#include <fstream>

using namespace std;

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

int n;
int fr[305];

int main()
{
    int i,ans=0;
    char x,j;
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>x;
        fr[x]++;
        for(j=x+1;j<='z';j++)
        {
            ans=fr[j]+ans;
        }
    }

    fout<<ans;
}