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.text.DecimalFormat;
import java.util.*;
import java.io.*;
import java.math.*;
public class Main {
	public static long mod= (long) (1e9 +7);
	static InputReader s;
	static PrintWriter out;
	static HashMap<Character,String> ans= new HashMap<>();
	public static void main(String args[]) {
		new Thread(null, new Runnable() {
			public void run() {
				try{
					solve();
					out.flush();
					out.close();
				}
				catch(Exception e){
					e.printStackTrace();
				}
			}
		}, "1", 1 << 26).start();
	}
	public static void solve() throws IOException{
		s= new InputReader(System.in);
		OutputStream outputStream= System.out;
		out= new PrintWriter(outputStream);
		int n= s.nextInt();
		int k= s.nextInt();
		int w[]= new int[n];
		for(int i=0;i<n;i++)
			w[i]= s.nextInt();
		long cnt=0;
		for(int i=0;i<n;i++){
			cnt+= Math.ceil(((double)w[i])/k);
		}
//		out.println(cnt);
		out.println((long)Math.ceil(cnt/2.0));
	}
	
	static long combinations(long n,long r){   // O(r)
		if(r==0 || r==n) return 1;
		r= Math.min(r, n-r);
		long ans=n;  // nC1=n;
		for(int i=1;i<r;i++){
			ans= ans*(n-i);
			ans= ans/(i+1);
		}
		return ans;	
	}
			
	static long modulo(long x,long y,long p)	
	{
		 long res = 1;
		 x = x % p;  
		 
	    while (y > 0)
	    {
	        if (y%2==1)
	            res = (res*x) % p;
	 
	        y = y>>1;
	        x = (x*x) % p;  
	    }
	    return res;
	}
	

	
	static HashSet<Integer> primeFactors(int n)   // O(sqrt(n))
	{
	    // Print the number of 2s that divide n
		HashSet<Integer> arr= new HashSet<>();
	    while (n%2 == 0)
	    {
	        arr.add(2);
	        n = n/2;
	    }
	 
	    // n must be odd at this point.  So we can skip one element (Note i = i +2)
	    for (int i = 3; i <= Math.sqrt(n); i = i+2)
	    {
	        // While i divides n, print i and divide n
	        while (n%i == 0)
	        {
	            arr.add(i);
	            n = n/i;
	        }
	    }
	    // This condition is to handle the case when n is a prime number
	    // greater than 2
	    if (n > 2)
	        arr.add(n);
	    
	    return arr;
	}
 
	public static long expo(long a, long b){
		if (b==0)
	        return 1;
	    if (b==1)
	        return a;
	    if (b==2)
	        return a*a;
 
	    if (b%2==0){
	            return expo(expo(a,(b/2)),2);
	    }
	    else{
	        return a*expo(expo(a,(b-1)/2),2);
	    }
	}
	static class Pair implements Comparable<Pair>
	{
		long a;
		int b;
//		int c;
		Pair(long aa, int bb)
		{
			a= aa;
			b= bb;
//			c=cc;
		}
		
		public int compareTo(Pair o) 
		{
			int ans = Long.compare(this.a,o.a);
			return ans;
//			return ans==0 ? -Integer.compare(this.b, o.b):ans;
//			if(Long.compare(this.x, o.x)!=0)
//				return -Long.compare(this.x, o.x);
//			else
//				return Long.compare(this.y, o.y);
//			return Integer.compare(this.x, o.x);
	//		return Long.compare(this.x, o.x);
		}
		
		public int hashCode() {
			int hu = (int) (a ^ (a >>> 32));
			int hv = (int) (b ^ (b >>> 32));
			return 31 * hu + hv;
		}
// 
		public boolean equals(Object o) {
			Pair other = (Pair) o;
			return a == other.a && b == other.b;
		}
		
		public String toString() {
			return "[x=" + a + ", y=" + b + "]";
		}
		
	}
	
	public static int[] sieve(int N){    // O(n*log(logn))
		int arr[]= new int[N+1];
		for(int i=2;i<=Math.sqrt(N);i++){
			if(arr[i]==0){
				for(int j= i*i;j<= N;j= j+i){
					arr[j]=1;
				}
			}
		}
		return arr;
		// All the i for which arr[i]==0 are prime numbers.
	}
	static int gcd(int a,int b){       // O(logn)
		if(b==0) return a;
		return gcd(b,a%b);
	}
	
	public static void debug(Object... o) {
		System.out.println(Arrays.deepToString(o));
	}
	
	static void catalan_numbers(int n) {
		long catalan[]= new long[n+1];
	    catalan[1] = 1;
	    for (int i = 2; i <= n; i++) {
	        for (int j = 1; j <= i - 1; j++) {
	            catalan[i] = catalan[i] + ((catalan[j]) * catalan[i - j]);
	        }
	    }
	}
	
	static final class InputReader{
        private final InputStream stream;
        private final byte[] buf=new byte[1024];
        private int curChar;
        private int Chars;
        public InputReader(InputStream stream){this.stream=stream;}
		private int read()throws IOException{
            if(curChar>=Chars){
                curChar=0;
                Chars=stream.read(buf);
                if(Chars<=0)
                    return -1;
            }
            return buf[curChar++];
        }
        public final int nextInt()throws IOException{return (int)nextLong();}
        public final long nextLong()throws IOException{
            int c=read();
            while(isSpaceChar(c)){
                c=read();
                if(c==-1) throw new IOException();
            }
            boolean negative=false;
            if(c=='-'){
                negative=true;
                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 negative?(-res):(res);
        }
        public final int[] nextIntBrray(int size)throws IOException{
            int[] arr=new int[size];
            for(int i=0;i<size;i++)arr[i]=nextInt();
            return arr;
        }
        public final String next()throws IOException{
            int c=read();
            while(isSpaceChar(c))c=read();
            StringBuilder res=new StringBuilder();
            do{
                res.append((char)c);
                c=read();
            }while(!isSpaceChar(c));
            return res.toString();
        }
        public final String nextLine()throws IOException{
            int c=read();
            while(isSpaceChar(c))c=read();
            StringBuilder res=new StringBuilder();
            do{
                res.append((char)c);
                c=read();
            }while(c!='\n'&&c!=-1);
            return res.toString();
        }
        private boolean isSpaceChar(int c){
            return c==' '||c=='\n'||c=='\r'||c=='\t'||c==-1;
        }
    }
}