#include<fstream>
#include<string>

using namespace std;

ifstream cin("litere.in");
ofstream cout("litere.out");

int n, i, j, rs=0;
string s;

int main()
{
    cin >> n;
    getline(cin,s);
    getline(cin,s);

    for (i=1;i<s.size();i++)
    {
        j=i;
        while (j && s[j]<s[j-1])swap(s[j],s[j-1]),rs++,j--;
    }
    cout << rs;
    return 0;
}