#include<fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
char S[10005], j;
int n, x, y, i, v[130];
int main ()
{
    fin>>n;
    fin>>S;
    x=0;
    v[S[0]-'a']=1;
    for(i=1;i<n;i++)
    {
        y=x;
        v[S[i]-'a']++;
        for(char j=S[i]-'a'+1;j<='z'-'a';j++)
        y=y+v[j];
        x=y;
    }
    fout<<y;
}