import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner koko = new Scanner(System.in);
        int n = koko.nextInt();
        int[] a = new int[n];
        int total =0;
        for(int i=0; i<n; i++){
            a[i]=koko.nextInt();
            total=total+a[i];
        }
        int idou = total;
        int c = 1;
        for(int i=1; i*i<=total; i+=2){
            int put=0;
            int rem=0;
            for(int j=0; j<(i-1)/2; j++){
                if(a[j]<=c){
                    put=put+c-a[j];
                }else{
                    rem=rem+a[j]-c;
                }
                c++;
            }
            for(int j=(i-1)/2; j<i-1; j++){
                if(a[j]<=c){
                    put=put+c-a[j];
                }else{
                    rem=rem+a[j]-c;
                }
                c--;
            }
            if(a[i-1]<=c){
                    put=put+c-a[i-1];
                }else{
                    rem=rem+a[i-1]-c;
                }
            for(int j=i; j<n; j++){
                rem=rem+a[j];
            }
            idou=Math.min(idou, Math.max(rem, put));
        }
        System.out.println(idou);
    }
}