#include <fstream>
using namespace std;ifstream f("litere.in");ofstream g("litere.out");char ch;int v[128],i,n,nr,j;int main(){f>>n;nr=0;for(i=1;i<=n;i++){f>>ch;v[ch]++;for(j=ch+1;j<='z';j++)nr=nr+v[j];}g<<nr;return 0;}