import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

auto rdsp(){return readln.splitter;}
void pick(R,T)(ref R r,ref T t){t=r.front.to!T;r.popFront;}
void readV(T...)(ref T t){auto r=rdsp;foreach(ref v;t)pick(r,v);}
void readA(T)(size_t n,ref T[]t){t=new T[](n);auto r=rdsp;foreach(ref v;t)pick(r,v);}

void main()
{
  int n; readV(n);
  int[] a; readA(n, a);

  foreach (i; 0..n) a[i] -= i+1;
  a.sort();

  auto b = n%2 == 1 ? a[n/2] : (a[n/2-1]+a[n/2])/2;
  auto r = 0L;
  foreach (i; 0..n) r += (a[i]-b).abs;

  writeln(r);
}
