#include<iostream>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
long long n,i,j,v[10005],k,x;
char v1[10005];
int main()
{
    cin>>n;
    for(i=1; i<=n; i++)
    {
        cin>>v1[i];
        v[i]=int(v1[i])-96;
    }
    for(i=2; i<=n; i++)
    {
        j=i-1;
        k=v[i];
        while(k<v[j])
        {
            v[j+1]=v[j];
            j--;
            x++;
        }
        v[j+1]=k;
    }
    cout<<x;
    return 0;

}