import java.io.IOException;
import java.util.Arrays;
import java.util.InputMismatchException;

public class TurnOffTheLights {
    public static void main(String[] args) {
        FasterScanner sc = new FasterScanner();
        
        int N = sc.nextInt();
        int K = sc.nextInt();
        long[] A = sc.nextLongArray(N);
        
        
        
        long best = Long.MAX_VALUE;
        
        int dx = 2 * K + 1;
        for (int i = 0; i <= K && i < N; i++) {
        	long cost = A[i];
        	for (int j = i + dx; ; j += dx) {
        		if (j >= N) {
        			if (j - N >= K) {
        				best = Math.min(best, cost);
        			}
        			break;
        		}
        		cost += A[j];
        	}
        }
        
        System.out.println(best);
        
        
        
        
//        long[][] dp = new long[N][K];
//        for (int i = 0; i < N; i++) {
//        	Arrays.fill(dp[i], Long.MAX_VALUE);
//        }
        
//        if (K + 1 >= N) {
//        	long best = Long.MAX_VALUE;
//        	for (long c : A) {
//        		best = Math.min(best, c);
//        	}
//        	System.out.println(best);
//        	return;
//        }
//        
//        long[] dp = new long[N];
//        Arrays.fill(dp, Long.MAX_VALUE);
//        Arrays.fill(dp, 0, K + 1, 0);
//        
//        for (int i = 0; i < N; i++) {
//        	long nextCost = dp[i] + A[i];
//        	for (int j = i; j <= i + K && j < N; j++) {
//        		dp[j] = Math.min(dp[j], nextCost);
//        	}
//            
////            System.out.println(Arrays.toString(dp));
//        }
//        
//        System.out.println(dp[N - 1]);
    }
    
	public static class FasterScanner {
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;

		public int read() {
			if (numChars == -1)
				throw new InputMismatchException();
			if (curChar >= numChars) {
				curChar = 0;
				try {
					numChars = System.in.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (numChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}

		public String nextLine() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndOfLine(c));
			return res.toString();
		}

		public String nextString() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		public long nextLong() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			long 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 int nextInt() {
			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 int[] nextIntArray(int n) {
	        int[] arr = new int[n];
	        for (int i = 0; i < n; i++) {
	            arr[i] = nextInt();
	        }
	        return arr;
	    }
        
		public long[] nextLongArray(int n) {
		    long[] arr = new long[n];
		    for (int i = 0; i < n; i++) {
		        arr[i] = nextLong();
		    }
		    return arr;
		}

	    private boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
	}
}