#include <fstream>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int n,s,k,i,j,fr[256],nr;
char ch,ch1,ch2;
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
    	f>>ch;
    	fr[ch]++;
    	for(j=ch+1;j<='z';j++)
    	{
    		nr+=fr[j];
    	}
    }
    g<<nr;
    return 0;
}