/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// AMAZON OA -26th August
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] arr = new int[n];
		for(int i=0; i<n; i++){
			arr[i] = sc.nextInt();
		}
		int k = sc.nextInt();
		System.out.println(getMinimumCost(arr, k));
	}
	public static int getMinimumCost(int[] arr, int k){
		int ones = 0;
		int min =Integer.MAX_VALUE;
		int cost =0;
		int r =0, l=0;
		while(r<arr.length){
			if(arr[r] == 1){
				ones++;
				cost++;
			}
			if(r-l+1 == k){
				min = Math.min(min, cost);
				cost -=arr[l];
				l++;
			}	
			r++;
		}	
		return (min*(min+1)/2)+ones-min;
	}
}

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5
1
1
1
0
1
4