import java.util.Scanner;

public class Main77 {
	public static void main(String[] args) {
		Main77 p = new Main77();
	}

	public Main77() {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[201];
		for(int i=0;i<n;i++)
			a[i] = sc.nextInt();
		solve(n, a);
	}

	public void solve(int n, int[] a) {
		int res = Integer.MAX_VALUE;
		int[] sum = new int[a.length];
		sum[0] = a[0];
		for(int i=sum.length-2;i>=0;i--)
			sum[i] += sum[i+1] + a[i];
		
		
		for(int i=0;i<a.length/2;i++){
			int cur = -1;
			int e = 0;
			int f = 0;
			for(int j=0;j<i+1;j++){
				if(a[j] - (j+1) > 0)
					e+=a[j] - (j+1);
				else
					f-=(a[j] - (j+1));
			}
				
			for(int j=i+1;j<i*2+1;j++){
				if(a[j] - (i*2+1-j) > 0)
					e+=a[j] - (i*2+1-j);
				else
					f-=(a[j] - (i*2+1-j));
			}

			if(e>=f)
				cur = e + sum[i*2+1];
			else if(f>=e && f-e <= sum[i*2])
				cur = e + sum[i*2+1];
			
			
			if(cur>=0){
				res = Math.min(res, cur);
			}
		}
		
		System.out.println(res);
		
	}

}