/*
TASK:psort
LANG:C++
*/
#include<cstdio>
#include<algorithm>
#include<vector>
#include<queue>
#include<math.h>
using namespace std;
int a[50001],n,b[4];
void vhod()
{
     int v[50001], i, c=0, r=0, l=0, k=0, j=0;
     scanf("%d",&n);
     for (i=1;i<=n;i++)
         scanf("%d",&a[i]);
     for ( i=1;i<=n;i++ )
         {
          if ( (v[i]=a[i]-i)!=0 )
             if ( v[i]>0 ) b[1]=++l;
                else b[2]=++r;
         }
          /*if ( a[i+1]>a[i] ) b[3]++;
             else b[4]++;
         }
     if ( (v=a[n]-n)!=0 )
             if ( v>0 ) b[1]=++l;
                else b[2]=++r;*/
     l>r?c=l:c=r;
     for ( i=1;i<=n;i++ )
         {
          if((int)(fabs((double)(v[i])))>c)k++;
          if(v[i]==(v[n-i+1]*-1))j++;
         }
     /*sort(&b[0],&b[0]+5);
     if ( r==l ) c=l;
        else c=l>r?l:r;
     if (l==r&&r==0) c=0;
        else c=b[4];*/
     k/=2;
     if(j==n&&l==(n/2))c=j-1;
     else c+=k;
     printf("%d\n",c);
}
int main ()
{
    vhod();
    //system("pause");
    return 0;
}
