#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int main()
{

    int w[26]={1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26};
    int n,cnt=0;
    bool t=true;
    fin>>n;
    char v[n];
    for(int i=0;i<n;i++)
    {
        fin>>v[i];
    }
    while(t==true)
    {
        t=false;
        for(int i=0;i<n-1;i++)
        {
            if(w[v[i]-97]>w[v[i+1]-97])
            {
                swap(v[i+1],v[i]);
                cnt++;
            }
        }
        for(int i=0;i<n-1;i++)
        {
            if(w[v[i]-97]>w[v[i+1]-97])
            {
                t=true;
            }
        }
    }
    fout<<cnt;
    return 0;
}