#include<fstream>
#include<cstring>
#define maxn 10007
using namespace std;
ifstream fi("litere.in");
ofstream fo("litere.out");

int i,j,n,a[150],sol=0;
char s[maxn];

int main(){
    fi>>n; fi>>s;
    
    for(i=97;i<123;i++) a[i]=0;
    
    for(i=0;i<n;i++){
                     a[s[i]+0]++;
                     for(j=s[i]+1;j<123;j++) sol+=a[j];
                    }

    fo<<sol;
    
    fi.close();
    fo.close();
    return 0;
}