Problem:
Score : 400 points

Problem Statement

Snuke has a large collection of cards. Each card has an integer between 1 and N, inclusive, written on it.
He has A_i cards with an integer i.

Two cards can form a pair if the absolute value of the difference of the integers written on them is at most 1.

Snuke wants to create the maximum number of pairs from his cards, on the condition that no card should be used in multiple pairs. Find the maximum number of pairs that he can create.

Constraints

1 ≦ N ≦ 10^5

0 ≦ A_i ≦ 10^9 (1 ≦ i ≦ N)

All input values are integers.

Input

The input is given from Standard Input in the following format:

N
A_1
:
A_N

Output

Print the maximum number of pairs that Snuke can create.

Sample Input 1

4
4
0
3
2

Sample Output 1

4

For example, Snuke can create the following four pairs: (1,1),(1,1),(3,4),(3,4).

Sample Input 2

8
2
0
1
6
0
8
2
1

Sample Output 2

9

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

/*************************************************************************
  >         File: Main.java
  >       Author: Riho.Yoshioka
  >         Mail: riho.yoshioka@yandex.com
  > Created Time: Sun 21 Aug 2016 08:07:38 PM CST
*************************************************************************/
import java.io.*;
import java.util.*;

public class Main {
  public static void main(String[] args) {
    InputStream inputStream = System.in;
    OutputStream outputStream = System.out;
    InputReader in = new InputReader(inputStream);
    PrintWriter out = new PrintWriter(outputStream);
    
    TaskB solver = new TaskB();
    solver.solve(1, in, out);
    out.close();
  }

  static class TaskB {
    public void solve(int testNumber, InputReader in, PrintWriter out) {
      int n = in.nextInt();
      int[] a = new int[n + 1];
      long[][] dp = new long[n + 1][2];
      for (int i = 0; i < n; i++) {
        a[i + 1] = in.nextInt();
      }
      for (int i = 0; i <= n; i++) {
        dp[i][0] = dp[i][1] = -1;
      }
      dp[0][0] = 0;
      for (int i = 1; i <= n; i++) {
        if (dp[i - 1][0] >= 0) {
          dp[i][0] = dp[i - 1][0] + a[i] / 2;
          if (a[i] > 0) dp[i][1] = dp[i - 1][0] + (a[i] - 1) / 2;
        }
        if (dp[i - 1][1] >= 0) {
          dp[i][0] = Math.max(dp[i][0], dp[i - 1][1] + a[i] / 2);
          if (a[i] > 1) dp[i][1] = Math.max(dp[i][1], dp[i - 1][1] + (a[i] - 2) / 2 + 1);
        }
      }
      long ans = Math.max(dp[n][0], dp[n][1]);
      out.println(ans);
    }
  }

  static class InputReader {
    public BufferedReader reader;
    public StringTokenizer tokenizer;

    public InputReader(InputStream stream) {
      reader = new BufferedReader(new InputStreamReader(stream), 32768);
      tokenizer = null;
    }

    public String next() {
      while (tokenizer == null || !tokenizer.hasMoreTokens()) {
        try {
          tokenizer = new StringTokenizer(reader.readLine());
        } catch (IOException e) {
          throw new RuntimeException(e);
        }
      }
      return tokenizer.nextToken();
    }

    public int nextInt() {
      return Integer.parseInt(next());
    }

    public long nextLong() {
      return Long.parseLong(next());
    }

    public double nextDouble() {
      return Double.parseDouble(next());
    }
  }
}