#include<fstream.h>

long n,i,inj,aux,gata,nr,a[10000];
char c;

int main()
{
ifstream f("litere.in");
ofstream g("litere.out");
f>>n;
for (i=1;i<=n;i++)
    {
    f>>c;
    a[i]=c-'a'+1;
    }
for (i=1;i<n;i++)
    {
    if (a[i]>a[i+1])
       {
       aux=a[i];
       a[i]=a[i+1];
       a[i+1]=aux;
       nr++;
       i=0;
       }
    }

g<<nr<<'\n';
g.close();
return 0;
}