ABC102C.cpp

#include <iostream>
#include <algorithm>
#include <math.h>
using namespace std;
using ll = long long;
int main(int argc, char const *argv[]) {
  int n;
  cin>>n;
  ll a[n];
  for(int i=0;i<n;i++){
    ll tmp;
    cin>>tmp;
    a[i]=tmp-i;
  }
  ll  med1=0,med2=0,median=0,ans=0;
  sort(a,a+n);
  if(n%2==0){
    med1=a[n/2-1];
    med2=a[n/2];
    median=(med1+med2)/2;
  }else{
    median=a[n/2];
  }
  for(int i=0;i<n;i++){
    ans+=abs(a[i]-median);
  }
  cout<<ans<<endl;
  return 0;
}