#include<fstream>
#include<cstring>
using namespace std;
char s[10004]; short b[200];
int main(void){
    ifstream fin("litere.in");
    ofstream fout("litere.out");
    int n,i,j,l,k1=0,nr=0,k;
    fin>>n; fin>>s; l=strlen(s); fin.close();
     
    for(i=0;i<l;++i){
                 k=0;    
                 for(j=s[i]+1;j<='z';++j)    
                     if(b[j])k+=b[j];
                 nr+=k; 
                 b[s[i]]++;   
                     }         
    fout<<nr; 
 return 0;   
}