Polycarp likes arithmetic progressions. A sequence [a_1, a_2, ..., a_n] is called an arithmetic progression if for each i (1 ≤ i < n) the value a_{i+1} - a_i is the same. For example, the sequences [42], [5, 5, 5], [2, 11, 20, 29] and [3, 2, 1, 0] are arithmetic progressions, but [1, 0, 1], [1, 3, 9] and [2, 3, 1] are not.

It follows from the definition that any sequence of length one or two is an arithmetic progression.

Polycarp found some sequence of positive integers [b_1, b_2, ..., b_n]. He agrees to change each element by at most one. In the other words, for each element there are exactly three options: an element can be decreased by 1, an element can be increased by 1, an element can be left unchanged.

Determine a minimum possible number of elements in b which can be changed (by exactly one), so that the sequence b becomes an arithmetic progression, or report that it is impossible.

It is possible that the resulting sequence contains element equals 0.

Input

The first line contains a single integer n (1 ≤ n ≤ 100 000) — the number of elements in b.

The second line contains a sequence b_1, b_2, ..., b_n (1 ≤ b_i ≤ 10^{9}).

Output

If it is impossible to make an arithmetic progression with described operations, print -1. In the other case, print non-negative integer — the minimum number of elements to change to make the given sequence becomes an arithmetic progression. The only allowed operation is to add/to subtract one from an element (can't use operation twice to the same position).

Examples

Input

4
24 21 14 10


Output

3


Input

2
500 500


Output

0


Input

3
14 5 1


Output

-1


Input

5
1 3 6 9 12


Output

1

Note

In the first example Polycarp should increase the first number on 1, decrease the second number on 1, increase the third number on 1, and the fourth number should left unchanged. So, after Polycarp changed three elements by one, his sequence became equals to [25, 20, 15, 10], which is an arithmetic progression.

In the second example Polycarp should not change anything, because his sequence is an arithmetic progression.

In the third example it is impossible to make an arithmetic progression.

In the fourth example Polycarp should change only the first element, he should decrease it on one. After that his sequence will looks like [0, 3, 6, 9, 12], which is an arithmetic progression.
import java.io.*;
import java.util.*;

public class TaskD
{
	public static void main(String[] args)
	{
		new TaskD(System.in, System.out);
	}

	static class Solver implements Runnable
	{
		int n;
		int[] arr;
		InputReader in;
		PrintWriter out;

		void solve() throws IOException
		{
			n = in.nextInt();
			arr = in.nextIntArray(n);

			if (n == 1 || n == 2)
			{
				out.println(0);

				return;
			}

			int min = Integer.MAX_VALUE;
			int[] diff = new int[]{arr[1] - arr[0], arr[1] - arr[0] - 1, arr[1] - arr[0] + 1, arr[1] - 1 - arr[0],
					arr[1] - 1 - arr[0] - 1, arr[1] - 1 - arr[0] + 1, arr[1] + 1 - arr[0], arr[1] + 1 - arr[0] - 1,
					arr[1] + 1 - arr[0] + 1};

			for (int d : diff)
				min = Math.min(min, check(arr[0], d));

			for (int d : diff)
				min = Math.min(min, check(arr[0] - 1, d));

			for (int d : diff)
				min = Math.min(min, check(arr[0] + 1, d));

			if (min == Integer.MAX_VALUE)
				out.println(-1);
			else
				out.println(min);
		}

		int check(int min, int diff)
		{
			int[] temp = new int[n];

			temp[0] = min;

			for (int i = 1; i < n; i++)
				temp[i] = temp[i - 1] + diff;

//			System.out.println("min : " + min + ", diff : " + diff + ", temp : " + Arrays.toString(temp));

			int steps = 0;

			for (int i = 0; i < n; i++)
			{
				int abs = Math.abs(temp[i] - arr[i]);

				if (abs == 1)
					steps++;
				else if (abs > 1)
					return Integer.MAX_VALUE;
			}

			return steps;
		}

		void debug(Object... o)
		{
			System.err.println(Arrays.deepToString(o));
		}

		//		uncomment below line to change to BufferedReader
//		public Solver(BufferedReader in, PrintWriter out)
		public Solver(InputReader in, PrintWriter out)
		{
			this.in = in;
			this.out = out;
		}

		@Override public void run()
		{
			try
			{
				solve();
			}
			catch (IOException e)
			{
				e.printStackTrace();
			}
		}

	}

	static class InputReader
	{
		private InputStream stream;
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;

		public int read()
		{
			if (numChars == -1)
				throw new InputMismatchException();

			if (curChar >= numChars)
			{
				curChar = 0;
				try
				{
					numChars = stream.read(buf);
				}
				catch (IOException e)
				{
					throw new InputMismatchException();
				}
				if (numChars <= 0)
					return -1;
			}

			return buf[curChar++];
		}

		public int nextInt()
		{
			int c = read();

			while (isSpaceChar(c))
				c = read();

			int sgn = 1;

			if (c == '-')
			{
				sgn = -1;
				c = read();
			}

			int res = 0;

			do
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();

				res *= 10;
				res += c & 15;

				c = read();
			} while (!isSpaceChar(c));

			return res * sgn;
		}

		public int[] nextIntArray(int arraySize)
		{
			int array[] = new int[arraySize];

			for (int i = 0; i < arraySize; i++)
				array[i] = nextInt();

			return array;
		}

		public long nextLong()
		{
			int c = read();

			while (isSpaceChar(c))
				c = read();

			int sign = 1;

			if (c == '-')
			{
				sign = -1;

				c = read();
			}

			long result = 0;

			do
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();

				result *= 10;
				result += c & 15;

				c = read();
			} while (!isSpaceChar(c));

			return result * sign;
		}

		public long[] nextLongArray(int arraySize)
		{
			long array[] = new long[arraySize];

			for (int i = 0; i < arraySize; i++)
				array[i] = nextLong();

			return array;
		}

		public float nextFloat()
		{
			float result, div;
			byte c;

			result = 0;
			div = 1;
			c = (byte) read();

			while (c <= ' ')
				c = (byte) read();

			boolean isNegative = (c == '-');

			if (isNegative)
				c = (byte) read();

			do
			{
				result = result * 10 + c - '0';
			} while ((c = (byte) read()) >= '0' && c <= '9');

			if (c == '.')
				while ((c = (byte) read()) >= '0' && c <= '9')
					result += (c - '0') / (div *= 10);

			if (isNegative)
				return -result;

			return result;
		}

		public double nextDouble()
		{
			double ret = 0, div = 1;
			byte c = (byte) read();

			while (c <= ' ')
				c = (byte) read();

			boolean neg = (c == '-');

			if (neg)
				c = (byte) read();

			do
			{
				ret = ret * 10 + c - '0';
			} while ((c = (byte) read()) >= '0' && c <= '9');

			if (c == '.')
				while ((c = (byte) read()) >= '0' && c <= '9')
					ret += (c - '0') / (div *= 10);

			if (neg)
				return -ret;

			return ret;
		}

		public String next()
		{
			int c = read();

			while (isSpaceChar(c))
				c = read();

			StringBuilder res = new StringBuilder();

			do
			{
				res.appendCodePoint(c);

				c = read();
			} while (!isSpaceChar(c));

			return res.toString();
		}

		public String nextLine()
		{
			int c = read();

			StringBuilder result = new StringBuilder();

			do
			{
				result.appendCodePoint(c);

				c = read();
			} while (!isNewLine(c));

			return result.toString();
		}

		public boolean isNewLine(int c)
		{
			return c == '\n';
		}

		public boolean isSpaceChar(int c)
		{
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		public void close()
		{
			try
			{
				stream.close();
			}
			catch (IOException e)
			{
				e.printStackTrace();
			}
		}

		public InputReader(InputStream stream)
		{
			this.stream = stream;
		}

	}

	static class CMath
	{
		static long power(long number, long power)
		{
			if (number == 1 || number == 0 || power == 0)
				return 1;

			if (power == 1)
				return number;

			if (power % 2 == 0)
				return power(number * number, power / 2);
			else
				return power(number * number, power / 2) * number;
		}

		static long modPower(long number, long power, long mod)
		{
			if (number == 1 || number == 0 || power == 0)
				return 1;

			number = mod(number, mod);

			if (power == 1)
				return number;

			long square = mod(number * number, mod);

			if (power % 2 == 0)
				return modPower(square, power / 2, mod);
			else
				return mod(modPower(square, power / 2, mod) * number, mod);
		}

		static long moduloInverse(long number, long mod)
		{
			return modPower(number, mod - 2, mod);
		}

		static long mod(long number, long mod)
		{
			return number - (number / mod) * mod;
		}

		static int gcd(int a, int b)
		{
			if (b == 0)
				return a;
			else
				return gcd(b, a % b);
		}

		static long min(long... arr)
		{
			long min = arr[0];

			for (int i = 1; i < arr.length; i++)
				min = Math.min(min, arr[i]);

			return min;
		}

		static long max(long... arr)
		{
			long max = arr[0];

			for (int i = 1; i < arr.length; i++)
				max = Math.max(max, arr[i]);

			return max;
		}

		static int min(int... arr)
		{
			int min = arr[0];

			for (int i = 1; i < arr.length; i++)
				min = Math.min(min, arr[i]);

			return min;
		}

		static int max(int... arr)
		{
			int max = arr[0];

			for (int i = 1; i < arr.length; i++)
				max = Math.max(max, arr[i]);

			return max;
		}

	}

	static class Utils
	{
		static boolean nextPermutation(int[] arr)
		{
			for (int a = arr.length - 2; a >= 0; --a)
			{
				if (arr[a] < arr[a + 1])
				{
					for (int b = arr.length - 1; ; --b)
					{
						if (arr[b] > arr[a])
						{
							int t = arr[a];

							arr[a] = arr[b];
							arr[b] = t;

							for (++a, b = arr.length - 1; a < b; ++a, --b)
							{
								t = arr[a];
								arr[a] = arr[b];
								arr[b] = t;
							}

							return true;
						}
					}
				}
			}

			return false;
		}

	}

	public TaskD(InputStream inputStream, OutputStream outputStream)
	{
//		uncomment below line to change to BufferedReader
//		BufferedReader in = new BufferedReader(new InputStreamReader(inputStream));
		InputReader in = new InputReader(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		Thread thread = new Thread(null, new Solver(in, out), "TaskD", 1 << 29);

		try
		{
			thread.start();
			thread.join();
		}
		catch (InterruptedException e)
		{
			e.printStackTrace();
		}
		finally
		{
			in.close();
			out.flush();
			out.close();
		}
	}

}
