#include <stdio.h>
#include <stdlib.h>

double pow0_5(int i)
{
    if (i == 0)
    {
        return 0;
    }

    int counter = -1;
    double current = i, currentHalf = i;
    while (counter < currentHalf)
    {
        current = (current / 2) + (i / (2 * current));
        counter++;
        currentHalf /= 2;
    }

    return current;
}

int calculateLumeria(int i, int value)
{
    return (int)(i * pow0_5(i)) + value;
}

int findMaxOfD(int p[], int current, int length, int distance, int *w)
{
    int max = calculateLumeria(current, p[current]), maxIndex = current;
    for (int i = current + 1; (i < current + distance) && (i < length); i++)
    {
        int lumValue = calculateLumeria(i, p[i]);
        if (max < lumValue)
        {
            max = lumValue;
            maxIndex = i;
        }
    }

    (*w) = max;
    return maxIndex;
}

int maxRobotWeight(int p[], int n, int d)
{
    int maxW = calculateLumeria(0, p[0]), i = 0;

    while (i < n - 1)
    {
        int currentMax;
        i = findMaxOfD(p, i + 1, n, d, &currentMax);
        if (currentMax < maxW)
        {
            maxW = currentMax;
        }
    }

    return maxW;
}

int main(void)
{
    int n, d;
    if (scanf("%d", &n) != 1)
    {
        return 1;
    }
    int *p = (int *) malloc(sizeof(int) * n);
    if (!p)
    {
        return 1;
    }
    for (int i = 0; i < n; i++)
    {
        if (scanf("%d", p + i) != 1)
        {
            free(p);
            return 1;
        }
    }
    if (scanf("%d", &d) != 1)
    {
        free(p);
        return 1;
    }
    printf("%d", maxRobotWeight(p, n, d));
    free(p);
    return 0;
}