#include <bits/stdc++.h>

using namespace std;
ifstream fin ("litere.in");
ofstream fout ("litere.out");
int main()
{
   short int n,v[10001],c=0;
    char s[10001];
    fin>>n;
    fin>>s;
    for (int i=0; i<n; i++)
    {
        v[i]=s[i]+'0';

    }
    bool sortat;
    do
    {
        sortat=true;
        for(int i=0; i<n-1; i++)
            if(v[i]>v[i+1])
            {
                c++;
                int aux=v[i];
                v[i]=v[i+1];
                v[i+1]=aux;
                sortat=false;
            }
    }
    while(!sortat);
    fout<<c;
    return 0;
}