#include <bits/stdc++.h>
#define NMAX 10010
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
string s;
int n, ans, fr[33];
int main()
{
    fin>>n;
    fin>>s;

    for(int i=0;i<s.size();++i)
    {
        for(int j=s[i]-'a'+1;j<='z'-'a';++j)
            ans+=fr[j];

        ++fr[s[i]-'a'];
    }

    fout<<ans;
    return 0;
}