/*
TASK:psort
LANG:C++
*/
#include<cstdio>
#include<algorithm>
#include<vector>
#include<queue>
using namespace std;
int a[50000],n;
void vhod()
{
     int v, i, c=0, r=0, l=0;
     scanf("%d",&n);
     for (i=1;i<=n;i++)
         scanf("%d",&a[i]);
     for (i=1;i<=n;i++)
         if ( (v=a[i]-i)!=0 )
            if ( v>0 ) r++;
               else l++;
     if ( r==l ) c=l;
        else c=l>r?l:r;
     printf("%d\n",c);
}
int main ()
{
    vhod();
    //system("pause");
    return 0;
}
