import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.io.Closeable;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) throws Exception {
        Thread thread = new Thread(null, new TaskAdapter(), "", 1 << 27);
        thread.start();
        thread.join();
    }

    static class TaskAdapter implements Runnable {
        @Override
        public void run() {
            InputStream inputStream = System.in;
            OutputStream outputStream = System.out;
            FastInput in = new FastInput(inputStream);
            FastOutput out = new FastOutput(outputStream);
            TaskB solver = new TaskB();
            solver.solve(1, in, out);
            out.close();
        }
    }
    static class TaskB {
        public void solve(int testNumber, FastInput in, FastOutput out) {
            int n = in.readInt();
            long x = in.readInt();

            long[] coords = new long[n];
            for (int i = 0; i < n; i++) {
                coords[i] = in.readInt();
            }

            long ans = Long.MAX_VALUE;
            PreSum ps = new PreSum(coords);
            for (int i = n - 1; i >= 0; i--) {
                int k = n - i;
                boolean valid = true;
                long localAns = k * x + 5 * ps.intervalSum(i, n - 1);
                int pick = 1;
                for (int j = i - 1; j >= 0; j -= k, pick++) {
                    int r = j;
                    int l = j - k + 1;
                    l = Math.max(l, 0);
                    long sum = ps.intervalSum(l, r);

                    valid = valid && !DigitUtils.isMultiplicationOverflow(sum, (2 * pick + 3), Long.MAX_VALUE);
                    valid = valid && !DigitUtils.isPlusOverflow(localAns, sum * (2 * pick + 3));
                    localAns += sum * (2 * pick + 3);
                }

                if (valid) {
                    ans = Math.min(localAns, ans);
                }
            }

            out.println(ans + n * x);
        }

    }
    static class PreSum {
        private long[] pre;

        public PreSum(long[] a) {
            int n = a.length;
            pre = new long[n];
            pre[0] = a[0];
            for (int i = 1; i < n; i++) {
                pre[i] = pre[i - 1] + a[i];
            }
        }

        public PreSum(int[] a) {
            int n = a.length;
            pre = new long[n];
            pre[0] = a[0];
            for (int i = 1; i < n; i++) {
                pre[i] = pre[i - 1] + a[i];
            }
        }

        public long intervalSum(int l, int r) {
            if (l == 0) {
                return pre[r];
            }
            return pre[r] - pre[l - 1];
        }

    }
    static class DigitUtils {
        private static final long[] DIGIT_VALUES = new long[19];
        static {
            DIGIT_VALUES[0] = 1;
            for (int i = 1; i < 19; i++) {
                DIGIT_VALUES[i] = DIGIT_VALUES[i - 1] * 10;
            }
        }

        private DigitUtils() {}

        public static boolean isMultiplicationOverflow(long a, long b, long limit) {
            if (limit < 0) {
                limit = -limit;
            }
            if (a < 0) {
                a = -a;
            }
            if (b < 0) {
                b = -b;
            }
            if (a == 0 || b == 0) {
                return false;
            }
            return a > limit / b;
        }

        public static boolean isPlusOverflow(long a, long b) {
            if (Long.signum(a) != Long.signum(b)) {
                return false;
            }
            if (a < 0) {
                return a + b > 0;
            } else {
                return a + b < 0;
            }
        }

    }
    static class FastInput {
        private final InputStream is;
        private byte[] buf = new byte[1 << 13];
        private int bufLen;
        private int bufOffset;
        private int next;

        public FastInput(InputStream is) {
            this.is = is;
        }

        private int read() {
            while (bufLen == bufOffset) {
                bufOffset = 0;
                try {
                    bufLen = is.read(buf);
                } catch (IOException e) {
                    bufLen = -1;
                }
                if (bufLen == -1) {
                    return -1;
                }
            }
            return buf[bufOffset++];
        }

        public void skipBlank() {
            while (next >= 0 && next <= 32) {
                next = read();
            }
        }

        public int readInt() {
            int sign = 1;

            skipBlank();
            if (next == '+' || next == '-') {
                sign = next == '+' ? 1 : -1;
                next = read();
            }

            int val = 0;
            if (sign == 1) {
                while (next >= '0' && next <= '9') {
                    val = val * 10 + next - '0';
                    next = read();
                }
            } else {
                while (next >= '0' && next <= '9') {
                    val = val * 10 - next + '0';
                    next = read();
                }
            }

            return val;
        }

    }
    static class FastOutput implements AutoCloseable, Closeable {
        private StringBuilder cache = new StringBuilder(10 << 20);
        private final Writer os;

        public FastOutput(Writer os) {
            this.os = os;
        }

        public FastOutput(OutputStream os) {
            this(new OutputStreamWriter(os));
        }

        public FastOutput println(long c) {
            cache.append(c).append('\n');
            return this;
        }

        public FastOutput flush() {
            try {
                os.append(cache);
                os.flush();
                cache.setLength(0);
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
            return this;
        }

        public void close() {
            flush();
            try {
                os.close();
            } catch (IOException e) {
                throw new UncheckedIOException(e);
            }
        }

        public String toString() {
            return cache.toString();
        }

    }
}



Measured memory_bytes: 50988