#include <iostream>
#include <fstream>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int fr[30];
void solve()
{
    int sol=0,n,j,i; char c; f>>n; f.get();
    for(i=1;i<=n;i++)
    {
        f>>c; fr[c-'a']++;
        for(j=c+1;j<='z';j++)sol+=fr[j-'a'];
    }
    g<<sol;
}
int main()
{
    solve();
    return 0;
}