#include <fstream>
#include <algorithm>
#include <cmath>
#include <cstring>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
int n,i,v[10005],j;
char s[10005];
long long g;
int main()
{
    cin>>n;
    cin>>s;
    for(i=0;i<n;i++)
        v[i]=int(s[i]);
    for(i=0;i<n;i++)
        for(j=i+1;j<n;j++)
            if(v[i]>v[j])
        {
            swap(v[i],v[j]);
            g++;
        }
    cout<<g;
    return 0;
}