#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;}