#include<fstream>
#include<string>

using namespace std;

ifstream cin("litere.in");
ofstream cout("litere.out");
const int nmax = 10010;
int n, i, j, rs=0,N;
char s[nmax],p[nmax],c;

int main()
{
    cin >> n; N=n;
    for (i=0;i<n;i++)cin>>c, s[i]=c;
    for (i=0;i<26;i++)
   {
       char c = (char)('a'+i);
       int k=0,l=0;
       for (j=0;j<n;j++)if (s[j]==c) rs += (j-l), l++, N--;
         else p[k++] = s[j];
    n=N;
      for (j=0;j<N;j++) s[j]=p[j];
    }

    cout << rs;
    return 0;
}