#include <bits/stdc++.h>

using namespace std;

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

string sortat[1000];

int main()
{
    int n,cnt=0;
    string s;
    in >> n >> s;
    string s2=s;
    sort (s.begin(),s.end());
    for (int i=0; i<s.size(); i++){
        for (int j=0; j<i; j++){
            if (s[i]>s2[j]){
                cnt++;
                swap(s[i],s[j]);
            }
        }
    }
    out << cnt;
    return 0;
}