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: Time Limit Exceeded)

import java.util.*;
import java.io.*;

public class Main {

    int[] input;
//    int[] a;

    void solve() {
	int N = sc.nextInt();
	input = sc.nextIntArray(N);

	ArrayList<Integer> a = new ArrayList<Integer>();

	for (int i = 0; i < input.length; i++) {
	    for (int ii = 0; ii < input[i]; ii++) {
		a.add(i+1);
	    }
	}

	int ans = 0;
	for (int i = 1; i < a.size(); i++) {
	    if (a.get(i)-a.get(i-1) <= 1) {
		ans++;
		i++;
//		System.out.println(a[i] + " " + a[i-1]);
	    }
	}
	System.out.println(ans);

    }

    void print(int[] a) {
	out.print(a[0]);
	for (int i = 1; i < a.length; i++) out.print(" " + a[i]);
	out.println();
    }

    public static void main(String[] args) throws Exception {
	new Main().run();
    }

    MyScanner sc = null;
    PrintWriter out = null;
    public void run() throws Exception {
	sc = new MyScanner(System.in);
	out = new PrintWriter(System.out);
	for (;sc.hasNext();) {
	    solve();
	    out.flush();
	}
	out.close();
    }

    class MyScanner {
	String line;
	BufferedReader reader;
	StringTokenizer tokenizer;

	public MyScanner(InputStream stream) {
	    reader = new BufferedReader(new InputStreamReader(stream));
	    tokenizer = null;
	}
	public void eat() {
	    while (tokenizer == null || !tokenizer.hasMoreTokens()) {
		try {
		    line = reader.readLine();
		    if (line == null) {
			tokenizer = null;
			return;
		    }
		    tokenizer = new StringTokenizer(line);
		} catch (IOException e) {
		    throw new RuntimeException(e);
		}
	    }
	}
	public String next() {
	    eat();
	    return tokenizer.nextToken();
	}
	public String nextLine() {
	    try {
		return reader.readLine();
	    } catch (IOException e) {
		throw new RuntimeException(e);
	    }
	}
	public boolean hasNext() {
	    eat();
	    return (tokenizer != null && tokenizer.hasMoreElements());
	}
	public int nextInt() {
	    return Integer.parseInt(next());
	}
	public long nextLong() {
	    return Long.parseLong(next());
	}
	public double nextDouble() {
	    return Double.parseDouble(next());
	}
	public int[] nextIntArray(int n) {
	    int[] a = new int[n];
	    for (int i = 0; i < n; i++) a[i] = nextInt();
	    return a;
	}
    }
}