#include<bits/stdc++.h>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
vector <int> v;
int x, i, number, n, j;
char s;
int main(){
fin>>n;
v.push_back(0);
for(i=1;i<=n;i++){
    fin>>s;
    v.push_back(s-'a');
}
for(i=1;i<=n;i++){
    for(j=1;j<n;j++){
        if(v[j] > v[j+1]){
            swap(v[j], v[j+1]);
            number++;
        }
    }
}
fout<<number;
return 0;
}