#include <fstream>
using namespace std;ifstream f("litere.in");ofstream g("litere.out");char c;int v[27];int N;int main(){int s=0;f>>N;f.get();for(int i=0;i<N;++i){f>>c;c-='a';for(int j=c+1;j<26;++j){s+=v[j];}v[c]++;}g<<s;}