You are given a Young diagram. 

Given diagram is a histogram with n columns of lengths a_1, a_2, …, a_n (a_1 ≥ a_2 ≥ … ≥ a_n ≥ 1).

<image> Young diagram for a=[3,2,2,2,1].

Your goal is to find the largest number of non-overlapping dominos that you can draw inside of this histogram, a domino is a 1 × 2 or 2 × 1 rectangle.

Input

The first line of input contain one integer n (1 ≤ n ≤ 300 000): the number of columns in the given histogram.

The next line of input contains n integers a_1, a_2, …, a_n (1 ≤ a_i ≤ 300 000, a_i ≥ a_{i+1}): the lengths of columns.

Output

Output one integer: the largest number of non-overlapping dominos that you can draw inside of the given Young diagram.

Example

Input


5
3 2 2 2 1


Output


4

Note

Some of the possible solutions for the example:

<image> <image>
import java.util.*;
import java.io.*;
import java.math.BigInteger;
 
public class Main implements Runnable {
  static Scanner in;
  // static FastReader in;
  static PrintWriter out;
  static int[][] dirs8 = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}, {1, 1}, {-1, -1}, {-1, 1}, {1, -1}};
  static int[][] dirs = {{0, -1}, {1, 0}, {0, +1}, {-1, 0}};
 
  static int[][] E;
  // static List<int[]>[] adj;
  static List<Integer>[] adj;
  static int[] vis;
 
  static long INFL = (long) 1e15 + 7;
  static int INF = (int) 1e7 + 9;
 
  static int T, n, m, k, count;
  static boolean flag;
  static int[] a, b;
  static int[][] matrix;
 
  static void solve() {
    n = in.nextInt();
    a = new int[n];
    long sum = 0;
    for (int i = 0; i < n; i++) {a[i] = in.nextInt(); sum += a[i];}
    int w = 0, b = 0;
    for (int i = 0; i < n; i++) {
      if (i % 2 == 0 && a[i] % 2 == 1) {w++;}
      if (i % 2 == 1 && a[i] % 2 == 1) {b++;}
    }
    long base = (sum - w - b) / 2;
    out.println(Math.min(w, b) + base);
 
  }
 
  
  public static void main(String[] args) throws Exception{
 
    in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
    // in = new FastReader();
    out = new PrintWriter(System.out);
 
    // int T = in.nextInt();
    // for (int tt = 1; tt <= T; tt++) {
    //   // Thread t = new Thread(null, new Main(), "Main", 1 << 28);
    //   // t.start();
    //   // t.join();
    //   solve();
    // }
 
    // // Thread t = new Thread(null, new Main(), "Main", 1 << 28);
    // // t.start();
    // // t.join();
    solve();
 
    out.flush();
    out.close();
  }
 
 
 
 
  @Override
  public void run() {solve();}
 
  static class FastReader {
    public BufferedReader br; 
    StringTokenizer st; 
    public FastReader() {br = new BufferedReader(new InputStreamReader(System.in));} 
    String next() { 
      while (st == null || !st.hasMoreElements()) { 
          try {st = new StringTokenizer(br.readLine());} 
          catch (IOException e) {e.printStackTrace();} 
      } 
      return st.nextToken(); 
    } 
    int nextInt() {return Integer.parseInt(next());} 
    long nextLong() {return Long.parseLong(next());} 
    double nextDouble() {return Double.parseDouble(next());} 
    String nextLine() {
      String str = ""; 
        try {str = br.readLine();} 
        catch (IOException e) {e.printStackTrace();} 
        return str; 
    }
  }
} 