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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include "bits/stdc++.h"
using namespace std;

//諸機能
#pragma region MACRO 
#define putans(x)  cerr << "answer: " ; cout << (x) << endl
#define dputans(x) cerr << "answer: "; cout << setprecision(10) << (double)(x) << endl
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define RREP(i,a,n) for(int i=(int)(n-1); i>= a; i--)
#define rep(i,n) REP(i,0,n)
#define rrep(i,n) RREP(i,0,n)
#define all(a) begin((a)),end((a))
#define mp make_pair
#define exist(container, n) ((container).find((n)) != (container).end())
#define substr(s,i,l) string((s), (i), (l))
#define tos(x) (to_string((x)))
#ifdef _DEBUG //ファイルからテストデータを読み込む
std::ifstream ifs("data.txt");
#define input ifs >>
#else //いい感じにやる
#define input cin >>
#endif
#pragma endregion
//デバッグなどの支援
#pragma region CODING_SUPPORT
#define dbg(var0) { std::cerr << ( #var0 ) << "=" << ( var0 ) << endl; }
#define dbg2(var0, var1) { std::cerr << ( #var0 ) << "=" << ( var0 ) << ", "; dbg(var1); }
#define dbg3(var0, var1, var2) { std::cerr << ( #var0 ) << "=" << ( var0 ) << ", "; dbg2(var1, var2); }
#define dbgArray(a,n) {std::cerr << (#a) << "=";  rep(i,n){std::cerr <<(a[i])<<",";} cerr<<endl;} 
#ifndef _DEBUG
#define dbg1 {}
#define dbg2 {}
#define dbg3 {}
#define dbgArray {}
#endif 
#pragma endregion 
//typedef（書き換える、書き足す可能性ある）
#pragma region TYPE_DEF
typedef long long ll;
typedef pair<int, int> pii; typedef pair<string, string> pss; typedef pair<int, string>pis;
typedef vector<string> vs; typedef vector<int> vi;
#pragma endregion
//諸々の定数(書き換える可能性ある)
#pragma region CONST_VAL
#define PI (2*acos(0.0))
#define EPS (1e-9)
#define MOD (ll)(1e9 + 7)
#define INF (ll)(1e9)
#pragma endregion

int main() {
	int n; input n;
	vi a;
	ll ans = 0;
	rep(i, n) {
		int k; input k;
		a.push_back(k);
	}
	a.push_back(0);
	rep(i, n) {
		ans += a[i] / 2;
		a[i] -= ( a[i] / 2 ) * 2;
		if (a[i]  % 2 == 1 && a[i+1] > 0) {
			ans++;
			a[i]--;
			a[i + 1]--;
		}
	}
	putans(ans);
	return 0;
}