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. 

 //   * * * its fun to do the impossible * * *   //
 import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.InputStreamReader;
 import java.util.*;
  
 
 public class AnastasiaAndPebbles {
    
     public static void main(String[] args) {
        FastScanner sc = new FastScanner();
        int n = sc.nextInt();
        int k = sc.nextInt();
        
        int w[] = sc.readArray(n);

        double count = 0;

        for(int e : w){
            count += Math.ceil((double)e/(double)k);
        }
        count /= 2;

        long res = 0;
        res = (long)Math.ceil(count);

        System.out.println(res);
     }
 
 
 
 
 
 
  
     // Use this instead of Arrays.sort() on an array of ints. Arrays.sort() is n^2
     // worst case since it uses a version of quicksort. Although this would never
     // actually show up in the real world, in codeforces, people can hack, so
     // this is needed.
     static void ruffleSort(int[] a) {
         //ruffle
         int n=a.length;
         Random r=new Random();
         for (int i=0; i<a.length; i++) {
             int oi=r.nextInt(n), temp=a[i];
             a[i]=a[oi];
             a[oi]=temp;
         }
         
         //then sort
         Arrays.sort(a);
     }
     
     // Use this to input code since it is faster than a Scanner
     static class FastScanner {
         BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
         StringTokenizer st=new StringTokenizer("");
         String next() {
             while (!st.hasMoreTokens())
                 try {
                     st=new StringTokenizer(br.readLine());
                 } catch (IOException e) {
                     e.printStackTrace();
                 }
             return st.nextToken();
         }
         
         int nextInt() {
             return Integer.parseInt(next());
         }
 
         double nextDouble() {
             return Double.parseDouble(next());
         }
  
         String str = "";
  
         String nextLine() {
             try {
                 str = br.readLine();
             } catch (IOException e) {
                 e.printStackTrace();
             }
             return str;
         }
  
         int[] readArray(int n) {
             int[] a = new int[n];
             for (int i = 0; i < n; i++)
                 a[i] = nextInt();
             return a;
         }
  
         long[] readLongArray(int n) {
             long[] a = new long[n];
             for (int i = 0; i < n; i++)
                 a[i] = nextLong();
             return a;
         }
         long nextLong() {
             return Long.parseLong(next());
         }
     }
     // use this to find the index of any element in the array +1 /// 
     // returns an array that corresponds to the index of the i+1th in the array a[]
     // runs only for array containing different values enclosed btw 0 to n-1
     static int[] indexOf(int[] a) {
         int[] toRet=new int[a.length];
         for (int i=0; i<a.length; i++) {
             toRet[a[i]]=i+1;
         }
         return toRet;
     }
 
     static int gcd(int a, int b) {
         if (b==0) return a;
         return gcd(b, a%b);
     }
 
     //generates all the prime numbers upto n
     static void sieveOfEratosthenes(int n , ArrayList<Integer> al)
     {
         boolean prime[] = new boolean[n + 1];
         for (int i = 0; i <= n; i++)
             prime[i] = true;
  
         for (int p = 2; p * p <= n; p++) 
         {
             if (prime[p] == true) 
             {
                 for (int i = p * p; i <= n; i += p)
                     prime[i] = false;
             }
         }
         for (int i = 2; i <= n; i++)
         {
             if (prime[i] == true)
                 al.get(i);
         }
     }
     static final int mod=100000000 + 7;
     
     //fastPow
     static long fastPow(long base, long exp) {
         if (exp==0) return 1;
         long half=fastPow(base, exp/2);
         if (exp%2==0) return mul(half, half);
         return mul(half, mul(half, base));
     }
 
     //multiply two long numbers
     static long mul(long a, long b) {
         return a*b%mod;
     }
 
     static int nCr(int n, int r)
     {
            return fact(n) / (fact(r) *
                 fact(n - r));
     }
     
     // Returns factorial of n
     static int fact(int n)
     {
         int res = 1;
         for (int i = 2; i <= n; i++)
             res = res * i;
         return res;
     }
      
     // to generate the lps array
     // lps means longest preffix that is also a suffix
     static void generateLPS(int lps[] , String p){
         int l = 0;
         int r = 1;
    
         while(l < p.length() && l < r && r < p.length()){
             if(p.charAt(l) == p.charAt(r)){
                 lps[r] = l + 1;
                 l++;
                 r++;
             }
             else{
                 if(l > 0)
                     l = lps[l - 1];
                 else
                     r++;    
             }
         }
     }
      
 
     //count sort --> it runs in O(n) time but compromises in space
     static ArrayList<Integer> countSort(int a[]){
         int max = Integer.MIN_VALUE;
         for(int i=0;i<a.length;i++){
             max = Math.max(max , a[i]);
         }
 
         int posfre[] = new int[max+1];
         boolean negPres = false;
         for(int i=0;i<a.length;i++){
             if(a[i]>=0){
                 posfre[a[i]]++;
             }
             else{
                 negPres = true;
             }
         }
         ArrayList<Integer> res = new ArrayList<>();
         if(negPres){
             int min = Integer.MAX_VALUE;
             for(int i=0;i<a.length;i++){
                 min = Math.min(min , a[i]);
             }
 
             int negfre[] = new int[-1*min+1];
             for(int i=0;i<a.length;i++){
                 if(a[i]<0){
                     negfre[-1*a[i]]++;
                 }
             }
 
             for(int i=min;i<0;i++){
                 for(int j=0;j<negfre[-1*i];j++){
                     res.add(i);
                 }
             }
             for(int i=0;i<=max;i++){
                 for(int j=0;j<posfre[i];j++){
                     res.add(i);
                 }
             }
             return res;
         }
         for(int i=0;i<=max;i++){
             for(int j=0;j<posfre[i];j++){
                 res.add(i);
             }
         }
         return res;
     }
 
     // a -> z == 97 -> 122
 
     // String.format("%.9f", ans) ,--> to get upto 9 decimal places , (ans is double)
     
     // write 
 }
 