Anastasia loves going for a walk in Central Uzhlyandian Park. But she became uninterested in simple walking, so she began to collect Uzhlyandian pebbles. At first, she decided to collect all the pebbles she could find in the park.

She has only two pockets. She can put at most k pebbles in each pocket at the same time. There are n different pebble types in the park, and there are wi pebbles of the i-th type. Anastasia is very responsible, so she never mixes pebbles of different types in same pocket. However, she can put different kinds of pebbles in different pockets at the same time. Unfortunately, she can't spend all her time collecting pebbles, so she can collect pebbles from the park only once a day.

Help her to find the minimum number of days needed to collect all the pebbles of Uzhlyandian Central Park, taking into consideration that Anastasia can't place pebbles of different types in same pocket.

Input

The first line contains two integers n and k (1 ≤ n ≤ 105, 1 ≤ k ≤ 109) — the number of different pebble types and number of pebbles Anastasia can place in one pocket.

The second line contains n integers w1, w2, ..., wn (1 ≤ wi ≤ 104) — number of pebbles of each type. 

Output

The only line of output contains one integer — the minimum number of days Anastasia needs to collect all the pebbles.

Examples

Input

3 2
2 3 4


Output

3


Input

5 4
3 1 8 9 7


Output

5

Note

In the first sample case, Anastasia can collect all pebbles of the first type on the first day, of second type — on the second day, and of third type — on the third day.

Optimal sequence of actions in the second sample case: 

  * In the first day Anastasia collects 8 pebbles of the third type. 
  * In the second day she collects 8 pebbles of the fourth type. 
  * In the third day she collects 3 pebbles of the first type and 1 pebble of the fourth type. 
  * In the fourth day she collects 7 pebbles of the fifth type. 
  * In the fifth day she collects 1 pebble of the second type. 
import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class main
{
	private static InputStream stream;
	private static byte[] buf = new byte[1024];
	private static int curChar;
	private static int numChars;
	private static SpaceCharFilter filter;
	private static PrintWriter pw;
	private static long mod = 1000000000 + 7;
	private static void soln()
	{
		int n = nextInt();
		int k = nextInt();
		int cnt = 0;
		//int[] arr = nextIntArray(n);
		for(int i = 0; i<n; i++) {
			int x = nextInt();
			if(x%k!=0)
				cnt++;
			cnt+=x/k;
		//	arr[i] = cnt;
		}
		cnt = (cnt+1)/2;
		pw.println(cnt);
	}
	public static class Segment {
		private int[] tree;
		private boolean[] lazy;
		private int size;
		private int n;
		private class node{
			private int a;
			private int b;
			private int c;
			public node(int x,int y,int z){
				a=x;
				b=y;
				c=z;
			}
		}
		public Segment(int n){
			//this.base=arr;
			int x = (int) (Math.ceil(Math.log(n) / Math.log(2)));
	        size = 2 * (int) Math.pow(2, x) - 1;
			tree=new int[size];
			lazy=new boolean[size];
			this.n=n;
			build(0,0,n-1);
		}
		public void build(int id,int l,int r) {
			if(l==r) {
				return;
			}
			int mid=(l+r)/2;
			build(2*id+1,l,mid);
			build(2*id+2,mid+1,r);
			tree[id]=tree[2*id+1]+tree[2*id+2];
		}
		public int query(int l,int r){
			return queryUtil(l,r,0,0,n-1);
		}
		private int queryUtil(int x,int y,int id,int l,int r){
			if(l>y || x>r) 
				return 0;
			if(x <= l && r<=y) {
				//if(!lazy[id])
					return tree[id];
				//else
				//	return tree[id][0];
			}
			int mid=l+(r-l)/2;
			shift(id);
			return queryUtil(x,y,2*id+1,l,mid)+queryUtil(x,y,2*id+2,mid+1,r);
		}
		public void update(int x,int y,int colour,int id,int l,int r){
			//System.out.println(l+" "+r+" "+x);
			if(x>r || y<l)
				return;
			if(x <= l && r<=y) {
				
				return;
			}
			int mid=l+(r-l)/2;
			shift(id);
			update(x,y,colour,2*id+1,l,mid);
			update(x,y,colour,2*id+2,mid+1,r);
			tree[id]=tree[2*id+1]+tree[2*id+2];
		}
		public void shift(int id) {
			
		}
	}

	public static void fft(double[] a, double[] b, boolean invert) {
		int count = a.length;
		for (int i = 1, j = 0; i < count; i++) {
			int bit = count >> 1;
			for (; j >= bit; bit >>= 1) j -= bit;
			j += bit;
			if (i < j) {
				double temp = a[i];
				a[i] = a[j];
				a[j] = temp;
				temp = b[i];
				b[i] = b[j];
				b[j] = temp;
			}
		}
		for (int len = 2; len <= count; len <<= 1) {
			int halfLen = len >> 1;
			double angle = 2 * Math.PI / len;
			if (invert) angle = -angle;
			double wLenA = Math.cos(angle), wLenB = Math.sin(angle);
			for (int i = 0; i < count; i += len) {
				double wA = 1, wB = 0;
				for (int j = 0; j < halfLen; j++) {
					double uA = a[i + j], uB = b[i + j];
					double vA = a[i + j + halfLen] * wA - b[i + j + halfLen] * wB;
					double vB = a[i + j + halfLen] * wB + b[i + j + halfLen] * wA;
					a[i + j] = uA + vA;
					b[i + j] = uB + vB;
					a[i + j + halfLen] = uA - vA;
					b[i + j + halfLen] = uB - vB;
					double nextWA = wA * wLenA - wB * wLenB;
					wB = wA * wLenB + wB * wLenA;
					wA = nextWA;
				}
			}
		}
		if (invert) {
			for (int i = 0; i < count; i++) {
				a[i] /= count;
				b[i] /= count;
			}
		}
	}
	static void multiply(long[][] a, long[][] b, long m)
	{
		int n = a.length;
		long[][] mul = new long[n][n];
		for (int i = 0; i < n; i++)
		{
			for (int j = 0; j < n; j++)
			{
				mul[i][j] = 0;
				for (int k = 0; k < n; k++)
					mul[i][j] = (mul[i][j] + (a[i][k] * b[k][j]) % m) % m;
			}
		}
		for (int i = 0; i < n; i++)
			for (int j = 0; j < n; j++)
				a[i][j] = mul[i][j];
	}

	private static long pow(long a, long b, long c)
	{
		if (b == 0)
			return 1;
		long p = pow(a, b / 2, c);
		p = (p * p) % c;
		return (b % 2 == 0) ? p : (a * p) % c;
	}

	private static long gcd(long n, long l)
	{
		if (l == 0)
			return n;
		return gcd(l, n % l);
	}
	public static void main(String[] args) throws Exception
	{
//		new Thread(null, new Runnable()
//		{
//			@Override
//			public void run()
//			{
//				/*try { 
//					InputReader(new FileInputStream("C:\\Users\\hardik\\Desktop\\B-large-practice.in")); 
//				} catch (FileNotFoundException e) { // TODO Auto-generated catch block
//					e.printStackTrace(); 
//				} */
//				InputReader(System.in);
//				pw = new PrintWriter(System.out);
//				/*try { 
//					pw=new PrintWriter(new FileOutputStream("C:\\Users\\hardik\\Desktop\\out.txt")); 
//				} catch (FileNotFoundException e) { // TODO Auto-generated catch block
//					e.printStackTrace();
//				}*/
//				soln();
//				pw.close();
//			}
//		}, "1", 1 << 26).start();
		
		/*try { 
			InputReader(new FileInputStream("C:\\Users\\hardik\\Desktop\\B-large-practice.in")); 
		} catch (FileNotFoundException e) { // TODO Auto-generated catch block
			e.printStackTrace(); 
		} */
		InputReader(System.in);
		pw = new PrintWriter(System.out);
		/*try { 
			pw=new PrintWriter(new FileOutputStream("C:\\Users\\hardik\\Desktop\\out.txt")); 
		} catch (FileNotFoundException e) { // TODO Auto-generated catch block
			e.printStackTrace();
		}*/
		soln();
		pw.close();
	}

	public static void InputReader(InputStream stream1)
	{
		stream = stream1;
	}

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

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

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

	private static 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;
	}

	private static 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;
	}

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

	private static 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();
	}

	private static int[] nextIntArray(int n)
	{
		int[] arr = new int[n];
		for (int i = 0; i < n; i++)
		{
			arr[i] = nextInt();
		}
		return arr;
	}

	private static long[] nextLongArray(int n)
	{
		long[] arr = new long[n];
		for (int i = 0; i < n; i++)
		{
			arr[i] = nextLong();
		}
		return arr;
	}

	private static void pArray(int[] arr)
	{
		for (int i = 0; i < arr.length; i++)
		{
			System.out.print(arr[i] + " ");
		}
		System.out.println();
		return;
	}

	private static void pArray(long[] arr)
	{
		for (int i = 0; i < arr.length; i++)
		{
			System.out.print(arr[i] + " ");
		}
		System.out.println();
		return;
	}

	private static boolean isSpaceChar(int c)
	{
		if (filter != null)
			return filter.isSpaceChar(c);
		return isWhitespace(c);
	}

	private static char nextChar()
	{
		int c = read();
		while (isSpaceChar(c))
			c = read();
		char c1 = (char) c;
		while (!isSpaceChar(c))
			c = read();
		return c1;
	}

	private interface SpaceCharFilter
	{
		public boolean isSpaceChar(int ch);
	}
}