import java.io.*;
import java.math.BigInteger;
import java.util.Arrays;
import java.util.InputMismatchException;

public class Main {

    public class CSegmentTree {
        long[] st;
        int n;

        public CSegmentTree(int n) {
            st = new long[4 * n];
            this.n = n;
        }

        void build(int i, int[] a, int r1, int r2) {
            if (r1 == r2) {
                st[i] = a[r1];
            } else {
                build(i * 2 + 1, a, r1, (r1 + r2) / 2);
                build(i * 2 + 2, a, (r1 + r2) / 2 + 1, r2);

                st[i] = st[i * 2 + 1] + st[i * 2 + 2];
            }
        }

        long query(int i, int ra, int rb, int r1, int r2) {
            if (ra > r2 || rb < r1) {
                return 0;
            }

            if (ra >= r1 && rb <= r2) {
                return st[i];
            }

            long p1 = query(i * 2 + 1, ra, (ra + rb) / 2, r1, r2);
            long p2 = query(i * 2 + 2, (ra + rb) / 2 + 1, rb, r1, r2);

            return p1 + p2;
        }

        long update(int i, int ra, int rb, int ind, long val) {
            if (ra == rb && rb == ind) {
                st[i] = val;
                return st[i];
            }

            if (ra > ind || rb < ind) {
                return st[i];
            }

            long p1 = update(i * 2 + 1, ra, (ra + rb) / 2, ind, val);
            long p2 = update(i * 2 + 2, (ra + rb) / 2 + 1, rb, ind, val);

            return st[i] = p1 + p2;
        }
    }

    public class SegmentTree {
        long[] st;
        int n;

        public SegmentTree(int n) {
            st = new long[4 * n];
            this.n = n;
        }

        void build(int i, int[] a, int r1, int r2) {
            if (r1 == r2) {
                st[i] = a[r1];
            } else {
                build(i * 2 + 1, a, r1, (r1 + r2) / 2);
                build(i * 2 + 2, a, (r1 + r2) / 2 + 1, r2);

                st[i] = st[i * 2 + 1] + st[i * 2 + 2];
            }
        }

        long query(int i, int ra, int rb, int r1, int r2) {
            if (ra > r2 || rb < r1) {
                return 0;
            }

            if (ra >= r1 && rb <= r2) {
                return st[i];
            }

            long p1 = query(i * 2 + 1, ra, (ra + rb) / 2, r1, r2);
            long p2 = query(i * 2 + 2, (ra + rb) / 2 + 1, rb, r1, r2);

            return p1 + p2;
        }

        long update(int i, int ra, int rb, int ind, long val) {
            if (ra == rb && rb == ind) {
                st[i] = val;
                return st[i];
            }

            if (ra > ind || rb < ind) {
                return st[i];
            }

            long p1 = update(i * 2 + 1, ra, (ra + rb) / 2, ind, val);
            long p2 = update(i * 2 + 2, (ra + rb) / 2 + 1, rb, ind, val);

            return st[i] = p1 + p2;
        }
    }

    class mini {
        int a;
        int b;
        int i;

        public mini(mini nm) {
            this.a = nm.a;
            this.b = nm.b;
            this.i = nm.i;
        }

        public mini(int a, int b, int i) {
            this.a = a;
            this.b = b;
            this.i = i;
        }
    }

    long max = (long) 1e9;

    public void solve(int testNumber, InputReader in, OutputWriter out) {
        int n = in.nextInt();

        long[] a = new long[n];

        for (int i = 0; i < n; i++) {
            a[i] = in.nextInt();
        }

        long lo = -max;
        long hi = max;
        while (lo <= hi) {
            long mid = (lo + hi) / 2;

            long val = cal(a, mid);
            long bef = cal(a, mid - 1);
            long aft = cal(a, mid + 1);

            if (bef >= val && aft >= val) {
                out.println(val);
                break;
            } else if (bef >= val && val >= aft) {
                lo = mid + 1;
            } else {
                hi = mid - 1;
            }
        }


    }

    long cal(long[] a, long b) {
        long sum = 0;
        for (int i = 0; i < a.length; i++) {
            sum += (long) Math.abs(a[i] - i - 1 - b);
        }
        return sum;
    }

    boolean isend(int[] b) {
        int n = b.length;

        for (int i = 0; i < n; i++) {
            if (b[i] != n - i)
                return false;
        }

        return true;
    }

    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        Main solver = new Main();
        solver.solve(1, in, out);
        out.close();
    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void println(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

        public void println(long i) {
            writer.println(i);
        }

    }

    public static class InputReader {
        private boolean finished = false;

        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

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

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

        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 peek() {
            if (numChars == -1) {
                return -1;
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    return -1;
                }
                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 - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public long nextLong() {
            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 nextString() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                if (Character.isValidCodePoint(c)) {
                    res.appendCodePoint(c);
                }
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        private String readLine0() {
            StringBuilder buf = new StringBuilder();
            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(nextString());
            } catch (NumberFormatException e) {
                throw new InputMismatchException();
            }
        }

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

        public double nextDouble() {
            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, nextInt());
                }
                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, nextInt());
                    }
                    if (c < '0' || c > '9') {
                        throw new InputMismatchException();
                    }
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }

        public boolean isExhausted() {
            int value;
            while (isSpaceChar(value = peek()) && value != -1) {
                read();
            }
            return value == -1;
        }

        public String next() {
            return nextString();
        }

        public InputReader.SpaceCharFilter getFilter() {
            return filter;
        }

        public void setFilter(InputReader.SpaceCharFilter filter) {
            this.filter = filter;
        }

        public int[] nextIntArray(int n) {
            int[] array = new int[n];
            for (int i = 0; i < n; ++i) array[i] = nextInt();
            return array;
        }

        public int[] nextSortedIntArray(int n) {
            int array[] = nextIntArray(n);
            Arrays.sort(array);
            return array;
        }

        public int[] nextSumIntArray(int n) {
            int[] array = new int[n];
            array[0] = nextInt();
            for (int i = 1; i < n; ++i) array[i] = array[i - 1] + nextInt();
            return array;
        }

        public long[] nextLongArray(int n) {
            long[] array = new long[n];
            for (int i = 0; i < n; ++i) array[i] = nextLong();
            return array;
        }

        public long[] nextSumLongArray(int n) {
            long[] array = new long[n];
            array[0] = nextInt();
            for (int i = 1; i < n; ++i) array[i] = array[i - 1] + nextInt();
            return array;
        }

        public long[] nextSortedLongArray(int n) {
            long array[] = nextLongArray(n);
            Arrays.sort(array);
            return array;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);
        }
    }
}


Measured memory_bytes: 24508