Problem:
Score : 400 points

Problem Statement

There are N squares aligned in a row.
The i-th square from the left contains an integer a_i.

Initially, all the squares are white.
Snuke will perform the following operation some number of times:

Select K consecutive squares. Then, paint all of them white, or paint all of them black. Here, the colors of the squares are overwritten.

After Snuke finishes performing the operation, the score will be calculated as the sum of the integers contained in the black squares.
Find the maximum possible score.

Constraints

1≤N≤10^5

1≤K≤N

a_i is an integer.

|a_i|≤10^9

Input

The input is given from Standard Input in the following format:

N K
a_1 a_2 ... a_N

Output

Print the maximum possible score.

Sample Input 1

5 3
-10 10 -10 10 -10

Sample Output 1

10

Paint the following squares black: the second, third and fourth squares from the left.

Sample Input 2

4 2
10 -10 -10 10

Sample Output 2

20

One possible way to obtain the maximum score is as follows:

Paint the following squares black: the first and second squares from the left.

Paint the following squares black: the third and fourth squares from the left.

Paint the following squares white: the second and third squares from the left.

Sample Input 3

1 1
-10

Sample Output 3

0

Sample Input 4

10 5
5 -4 -5 -8 -4 7 2 -4 0 7

Sample Output 4

17

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)


import java.io.*;
import java.util.StringTokenizer;

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

    static class T {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n  = in.nextInt(), k = in.nextInt();
            final int[] arr = new int[n];
            for (int i = 0; i < n; i++) {
                arr[i] = in.nextInt();
            }
            final long[] prefSum = new long[n];
            final long[] posSum = new long[n];
            for (int i = 0; i < n; i++) {
                if (i == 0) {
                    prefSum[i] = arr[i];
                    if (arr[i] > 0)
                        posSum[i] = arr[i];
                } else {
                    prefSum[i] = prefSum[i - 1] + arr[i];
                    posSum[i] = posSum[i - 1] + (arr[i] > 0 ? arr[i] : 0);
                }
            }

            long ans = 0;
            for (int i = 0; i < n; i++) {
                int j = i + k - 1;
                if (j >= n) break;
                long as = rangePosSum(0, i - 1, posSum) + rangePosSum(j + 1, n - 1, posSum);
                as = Math.max(as, as + rangePrefSum(i, j, prefSum));
                ans = Math.max(ans, as);
            }
            out.println(ans);

        }

        private long rangePosSum(int i, int j, long[] posSum) {
            if (j < i) return 0;
            if (i == 0) return posSum[j];
            return posSum[j] - posSum[i - 1];
        }

        private long rangePrefSum(int i, int j, long[] prefSum) {
            if (i == 0) return prefSum[j];
            return prefSum[j] - prefSum[i - 1];
        }
    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
}