On Bertown's main street n trees are growing, the tree number i has the height of ai meters (1 ≤ i ≤ n). By the arrival of the President of Berland these trees were decided to be changed so that their heights formed a beautiful sequence. This means that the heights of trees on ends (the 1st one and the n-th one) should be equal to each other, the heights of the 2-nd and the (n - 1)-th tree must also be equal to each other, at that the height of the 2-nd tree should be larger than the height of the first tree by 1, and so on. In other words, the heights of the trees, standing at equal distance from the edge (of one end of the sequence) must be equal to each other, and with the increasing of the distance from the edge by 1 the tree height must also increase by 1. For example, the sequences "2 3 4 5 5 4 3 2" and "1 2 3 2 1" are beautiful, and '1 3 3 1" and "1 2 3 1" are not. 

Changing the height of a tree is a very expensive operation, using advanced technologies invented by Berland scientists. In one operation you can choose any tree and change its height to any number, either increase or decrease. Note that even after the change the height should remain a positive integer, i. e, it can't be less than or equal to zero. Identify the smallest number of changes of the trees' height needed for the sequence of their heights to become beautiful.

Input

The first line contains integer n (1 ≤ n ≤ 105) which is the number of trees. The second line contains integers ai (1 ≤ ai ≤ 105) which are the heights of the trees.

Output

Print a single number which is the minimal number of trees whose heights will have to be changed for the sequence to become beautiful.

Examples

Input

3
2 2 2


Output

1


Input

4
1 2 2 1


Output

0
import java.io.*;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.InputMismatchException;
import java.util.List;

/**
 * @author Egor Kulikov (egor@egork.net)
 */
public class Task58C {
	@SuppressWarnings({"FieldCanBeLocal", "UnusedDeclaration"})
	private final InputReader in;
	private final PrintWriter out;
	private final boolean testMode;

	private void solve() {
		int treeCount = in.readInt();
		int[] trees = in.readIntArray(treeCount);
		int[] countVariant = new int[100001];
		for (int i = 0; i < treeCount; i++) {
			int distance = Math.min(i, treeCount - i - 1);
			if (trees[i] > distance)
				countVariant[trees[i] - distance]++;
		}
		int max = 0;
		for (int variant : countVariant)
			max = Math.max(variant, max);
		out.println(treeCount - max);
	}

	private static List<Test> createTests() {
		List<Test> tests = new ArrayList<Test>();
		tests.add(new Test("3\n" +
				"2 2 2", "1"));
		tests.add(new Test("4\n" +
				"1 2 2 1", "0"));
//		tests.add(new Test("", ""));
//		tests.add(new Test("", ""));
//		tests.add(new Test("", ""));
//		tests.add(new Test("", ""));
//		tests.add(new Test("", ""));
//		tests.add(new Test("", ""));
//		tests.add(new Test("", ""));
//		tests.add(new Test("", ""));
		return tests;
	}

	private void run() {
		//noinspection InfiniteLoopStatement
//		while (true)
//		int testCount = in.readInt();
//		for (int i = 0; i < testCount; i++)
		solve();
		exit();
	}

	private Task58C() {
		@SuppressWarnings({"UnusedDeclaration"})
		String id = getClass().getName().toLowerCase();
		//noinspection EmptyTryBlock
		try {
//			System.setIn(new FileInputStream(id + ".in"));
//			System.setOut(new PrintStream(new FileOutputStream(id + ".out")));
//			System.setIn(new FileInputStream("input.txt"));
//			System.setOut(new PrintStream(new FileOutputStream("output.txt")));
		} catch (Exception e) {
			throw new RuntimeException(e);
		}
		in = new StreamInputReader(System.in);
		out = new PrintWriter(System.out);
		testMode = false;
	}

	@SuppressWarnings({"UnusedParameters"})
	private static String check(String input, String result, String output) {
//		return strictCheck(result, output);
		return tokenCheck(result, output);
	}

	public static void main(String[] args) {
		if (args.length != 0 && args[0].equals("42"))
			test();
		else
			new Task58C().run();
	}

	private static void test() {
		List<Test> tests = createTests();
		int testCase = 0;
		for (Test test : tests) {
			System.out.print("Test #" + testCase + ": ");
			InputReader in = new StringInputReader(test.getInput());
			StringWriter out = new StringWriter(test.getOutput().length());
			long time = System.currentTimeMillis();
			try {
				new Task58C(in, new PrintWriter(out)).run();
			} catch (TestException e) {
				time = System.currentTimeMillis() - time;
				String checkResult = check(test.getInput(), out.getBuffer().toString(), test.getOutput());
				if (checkResult == null)
					System.out.print("OK");
				else
					System.out.print("WA (" + checkResult + ")");
				System.out.printf(" in %.3f s.\n", time / 1000.);
			} catch (Throwable e) {
				System.out.println("Exception thrown:");
				e.printStackTrace(System.out);
			}
			testCase++;
		}
	}

	private static String tokenCheck(String result, String output) {
		StringInputReader resultStream = new StringInputReader(result);
		StringInputReader outputStream = new StringInputReader(output);
		int index = 0;
		boolean readingResult = false;
		try {
			while (true) {
				readingResult = true;
				String resultToken = resultStream.readString();
				readingResult = false;
				String outputToken = outputStream.readString();
				if (!resultToken.equals(outputToken))
					return "'" + outputToken + "' expected at " + index + " but '" + resultToken + "' received";
				index++;
			}
		} catch (InputMismatchException e) {
			if (readingResult) {
				try {
					outputStream.readString();
					return "only " + index + " tokens received";
				} catch (InputMismatchException e1) {
					return null;
				}
			} else
				return "only " + index + " tokens expected";
		}
	}

	@SuppressWarnings({"UnusedDeclaration"})
	private static String strictCheck(String result, String output) {
		if (result.equals(output))
			return null;
		return "'" + output + "' expected but '" + result + "' received";
	}

	@SuppressWarnings({"UnusedDeclaration"})
	private static boolean isDoubleEquals(double expected, double result, double certainty) {
		return Math.abs(expected - result) < certainty || Math.abs(expected - result) < certainty * expected;
	}

	private Task58C(InputReader in, PrintWriter out) {
		this.in = in;
		this.out = out;
		testMode = true;
	}

	@SuppressWarnings({"UnusedDeclaration"})
	private void exit() {
		out.close();
		if (testMode)
			throw new TestException();
		System.exit(0);
	}

	private static class Test {
		private final String input;
		private final String output;

		private Test(String input, String output) {
			this.input = input;
			this.output = output;
		}

		public String getInput() {
			return input;
		}

		public String getOutput() {
			return output;
		}
	}

	@SuppressWarnings({"UnusedDeclaration"})
	private abstract static class InputReader {
		//		public InputReader(InputStream stream) {
//			this.stream = stream;
//		}
//
		public abstract 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 readInt() {
			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 - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}

		public long readLong() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			long res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}

		public String readString() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuffer res = new StringBuffer();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

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

		private String readLine0() {
			StringBuffer buf = new StringBuffer();
			int c = read();
			while (c != '\n' && c != -1) {
				if (c != '\r')
					buf.appendCodePoint(c);
				c = read();
			}
			return buf.toString();
		}

		public String readLine() {
			String s = readLine0();
			while (s.trim().length() == 0)
				s = readLine0();
			return s;
		}

		public String readLine(boolean ignoreEmptyLines) {
			if (ignoreEmptyLines)
				return readLine();
			else
				return readLine0();
		}

		public BigInteger readBigInteger() {
			try {
				return new BigInteger(readString());
			} catch (NumberFormatException e) {
				throw new InputMismatchException();
			}
		}

		public char readCharacter() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			return (char) c;
		}

		public double readDouble() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			double res = 0;
			while (!isSpaceChar(c) && c != '.') {
				if (c == 'e' || c == 'E')
					return res * Math.pow(10, readInt());
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			}
			if (c == '.') {
				c = read();
				double m = 1;
				while (!isSpaceChar(c)) {
					if (c == 'e' || c == 'E')
						return res * Math.pow(10, readInt());
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					m /= 10;
					res += (c - '0') * m;
					c = read();
				}
			}
			return res * sgn;
		}

		public int[] readIntArray(int size) {
			int[] array = new int[size];
			for (int i = 0; i < size; i++)
				array[i] = readInt();
			return array;
		}

		public long[] readLongArray(int size) {
			long[] array = new long[size];
			for (int i = 0; i < size; i++)
				array[i] = readLong();
			return array;
		}

		public double[] readDoubleArray(int size) {
			double[] array = new double[size];
			for (int i = 0; i < size; i++)
				array[i] = readDouble();
			return array;
		}

		public String[] readStringArray(int size) {
			String[] array = new String[size];
			for (int i = 0; i < size; i++)
				array[i] = readString();
			return array;
		}

		public char[][] readTable(int rowCount, int columnCount) {
			char[][] table = new char[rowCount][columnCount];
			for (int i = 0; i < rowCount; i++) {
				for (int j = 0; j < columnCount; j++)
					table[i][j] = readCharacter();
			}
			return table;
		}

		public void readIntArrays(int[]... arrays) {
			for (int i = 0; i < arrays[0].length; i++) {
				for (int j = 0; j < arrays.length; j++)
					arrays[j][i] = readInt();
			}
		}
	}

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

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

		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++];
		}
	}

	private static class StringInputReader extends InputReader {
		private Reader stream;
		private char[] buf = new char[1024];
		private int curChar, numChars;

		public StringInputReader(String stream) {
			this.stream = new StringReader(stream);
		}

		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++];
		}
	}

	private static class TestException extends RuntimeException {}
}
