#include <fstream>

using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
char x;
int n,v[200],i,s;

int main()
{
    f>>n;
    for(;n--;){
        f>>x;
        for(i=x+1;i<='z';i++) s+=v[i];
        v[x]++;
    }
    g<<s;
    return 0;
}