import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.math.BigInteger;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Egor Kulikov (egor@egork.net)
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskB {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.readInt();
            long drop = in.readInt();
            int[] x = in.readIntArray(n);
            FenwickTree tree = new FenwickTree(n);
            for (int i = 0; i < n; i++) {
                tree.add(i, x[i]);
            }
            long answer = Long.MAX_VALUE;
            BigInteger infty = BigInteger.valueOf(Long.MAX_VALUE);
            for (int i = 1; i <= n; i++) {
                BigInteger current = BigInteger.valueOf((n + i) * drop);
                for (int j = 0; j * i < n; j++) {
                    long by = j == 0 ? 5 : 2 * j + 3;
                    current =
                            current.add(BigInteger.valueOf(tree.get(Math.max(0, n - (j + 1) * i), n - j * i - 1))
                                    .multiply(BigInteger.valueOf(by)));
                }
                if (current.compareTo(infty) < 0) {
                    answer = Math.min(answer, current.longValue());
                }
            }
            out.printLine(answer);
        }

    }

    static class FenwickTree {
        private final long[] value;

        public FenwickTree(int size) {
            value = new long[size];
        }

        public long get(int from, int to) {
            if (from > to) {
                return 0;
            }
            return get(to) - get(from - 1);
        }

        private long get(int to) {
            to = Math.min(to, value.length - 1);
            long result = 0;
            while (to >= 0) {
                result += value[to];
                to = (to & (to + 1)) - 1;
            }
            return result;
        }

        public void add(int at, long value) {
            while (at < this.value.length) {
                this.value[at] += value;
                at = at | (at + 1);
            }
        }

    }

    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 close() {
            writer.close();
        }

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

    }

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

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

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

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

        }

    }
}



Measured memory_bytes: 204580