#include <bits/stdc++.h>

using namespace std;

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

int main()
{
    int n;
    in >> n;
    char s[n];
    in >> s;
    int ls = strlen(s);
    int interschimb = 1;
    int contor = 0;
    while(interschimb == 1)
    {
        interschimb = 0;
        for(int i = 0; i < ls - 1; i++)
        {
            if(s[i] - 'a' > s[i+1] - 'a'){
                interschimb = 1;
                swap(s[i], s[i+1]);
                contor++;
            }
        }
    }
    out << contor;
    return 0;
}