#include<stdio.h>

#define INPUT "litere.in"
#define OUTPUT "litere.out"
#define DIM 10001

int a[DIM];
int main()
{
    freopen(INPUT,"r",stdin);
    freopen(OUTPUT,"w",stdout);
    char ch;
    int n,i,asci,k=0,j,aux,ok;
    scanf("%d",&n);
    for(i=0; i<=n; ++i)
    {
             scanf("%c",&ch);
             asci=ch-'a';
             if(i!=0)
                     a[i]=asci;
    }
    do
    {
      ok=1;
      for(i=1;i<n;++i)
          if(a[i]>a[i+1])
          {
             ok=0;
             aux=a[i];
             a[i]=a[i+1];
             a[i+1]=aux;
             ++k;
          }
    }
    while(ok==0);
    printf("%d",k);
    return 0;
}