/******************************************************************************

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               Code, Compile, Run and Debug C++ program online.
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*******************************************************************************/

#include <bits/stdc++.h>
using namespace std;
int main() {
 ifstream f("litere.in");
 ofstream g("litere.out");
 int n, f[26] = {0};
 long long schimbari = 0 ,x;
 char c ;
 
 f >> n;
 
 for(i = 0; i < n; i++){
      f >> c;
      x = c - 'a';
      for(j = x + 1; j < 26; j++)
            schimbari += f[j];
     
     f[x]++;
     
 }
 
    g << schimbari;

  
  

    return 0;
}