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)


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

public class Main{
    static final Reader sc = new Reader();
    static final PrintWriter out = new PrintWriter(System.out,false);

    public static void main(String[] args) throws Exception {
    	int n = sc.nextInt();
    	long[] a = new long[n];
    	for(int i=0;i<n;i++){
    		a[i] = sc.nextLong();
    	}
    	long count = 0;
    	for(int i=0;i<n;i++){
    		count += a[i]/2;
    		a[i] -= a[i]/2*2;
    		if(i==0 && a[i]==1 && a[i+1]%2==1){
    			count++;
    			a[i]-=1;
    			a[i+1]-=1;
    		}
    		else if(i!=0 && a[i]==1 && a[i-1]==1){
    			count++;
    			a[i]-=1;
    			a[i-1]-=1;
    		}
    		else if(i!=0 && a[i]==1 && a[i+1]%2==1){
    			count++;
    			a[i]-=1;
    			a[i+1]-=1;
    		}
    	}
    	out.println(count);
    	out.flush();
        sc.close();
        out.close();
    }

    static void trace(Object... o) { System.out.println(Arrays.deepToString(o));}
}

class Reader {

    private final InputStream in;
    private final byte[] buf = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    public Reader() { this(System.in);}
    public Reader(InputStream source) { this.in = source;}

    private boolean hasNextByte() {
        if (ptr < buflen) return true;
        ptr = 0;
        try{
            buflen = in.read(buf);
        }catch (IOException e) {e.printStackTrace();}
        if (buflen <= 0) return false;
        return true;
    }

    private int readByte() { if (hasNextByte()) return buf[ptr++]; else return -1;}

    private boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}

    private void skip() { while(hasNextByte() && !isPrintableChar(buf[ptr])) ptr++;}

    public boolean hasNext() {skip(); return hasNextByte();}

    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        boolean minus = false;
        long num = readByte();

        if(num == '-'){
            num = 0;
            minus = true;
        }else if (num < '0' || '9' < num){
            throw new NumberFormatException();
        }else{
            num -= '0';
        }

        while(true){
            int b = readByte();
            if('0' <= b && b <= '9')
                num = num * 10 + (b - '0');
            else if(b == -1 || !isPrintableChar(b))
                return minus ? -num : num;
            else
                throw new NoSuchElementException();
        }
    }

    public int nextInt() {
        long num = nextLong();
        if (num < Integer.MIN_VALUE || Integer.MAX_VALUE < num)
            throw new NumberFormatException();
        return (int)num;
    }

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

    public char nextChar() {
        if (!hasNext()) throw new NoSuchElementException();
        return (char)readByte();
    }

    public String nextLine() {
        while (hasNextByte() && (buf[ptr] == '\n' || buf[ptr] == '\r')) ptr++;
        if (!hasNextByte()) throw new NoSuchElementException();

        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (b != '\n' && b != '\r') {
            sb.appendCodePoint(b);
            b = readByte();
        }

        return sb.toString();
    }

    public int[] nextIntArray(int n) {
        int[] res = new int[n];
        for (int i=0; i<n; i++) res[i] = nextInt();
        return res;
    }

    public char[] nextCharArray(int n) {
        char[] res = new char[n];
        for (int i=0; i<n; i++) res[i] = nextChar();
        return res;
    }

    public void close() {try{ in.close();}catch(IOException e){ e.printStackTrace();}};
}