#include<fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
char s[10005],v[30];
int n,x,i,j,y;
int main()
{
    fin>>n;
    fin>>s;

    x=0;
    v[s[0]-'a']=1;

    for(i=1;i<n;i++)
    {
        y=x;v[s[i]-'a']++;
        for(j=s[i]-'a'+1;j<'z'-'a';j++)
           y=y+v[j];
        x=y;
    }

    fout<<y;










    }