#include <bits/stdc++.h>

using namespace std;

ifstream fin ("litere.in");
ofstream fout ("litere.out");

int main()
{
    int n, i, nr=0, sir[30]={0}, j;
    fin>>n;
    char v[10001];
    for(i=1; i<=n; i++)
        fin>>v[i], sir[v[i]-96]++;

    int k=1;
    char *pinitial = &v[0];
    for(i=1; i<=26; i++)
    {
        while (sir[i])
        {
            char* p=strchr(v+k, char(96+i));
            int pozitie = p - pinitial;

            for(j=pozitie; j>k; j--)
                swap (v[j], v[j-1]), nr++;

            k++;
            sir[i]--;
        }
    }
    fout<<nr;
    return 0;
}