#include <fstream>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
int main()
{
    int n;
    cin >> n;
    char c;
    int v[30] = {0}; 
    int cnt=0;
    for(int i = 1; i <= n; i++)
    {
        cin >> c;
        int x = c - 'a' ; 
        for(int j=x+1;j<=26;j++)
            cnt +=v[j];
        v[x]++;  
        
    }

    cout<<cnt;
    

    return 0;
}