#include <bits/stdc++.h>
#define MOD 100000000
#define NMAX 100100
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
string s;
int n, nr, ok;
int main()
{
    fin>>n;
    fin>>s;

    ok=1;
    while(ok)
    {
        ok=0;
        for(int i=0;i+1<n;++i)
        {
            if(s[i]>s[i+1])
            {
                ok=1;
                ++nr;
                swap(s[i], s[i+1]);
                --i;
            }
        }
    }

    fout<<nr;
    return 0;
}