#include<fstream>
using namespace std;ifstream cin("litere.in");ofstream cout("litere.out");int n,i,nr,v[257],j;char q;int main(){cin>>n;for(i=1;i<=n;i++){cin>>q;v[q]++;for(j=q+1;j<='z';j++)nr=nr+v[j];}cout<<nr;return 0;}