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: Wrong Answer)

#include <bits/stdc++.h>

#define debug(x) cout <<"DEBUG"<<" "<< #x << ":" << x << '\n'
#define rep(i,n) for(int_fast32_t i = 0; i < ((int_fast32_t)(n)); i++)   // 0-indexed昇順
#define rep1(i,n) for(int_fast32_t i = 1; i <= ((int_fast32_t)(n)); i++) // 1-indexed昇順
#define rrep(i,n) for(int_fast32_t i = ((int_fast32_t)(n)-1); i >= 0; i--)  // 0-indexed降順
#define rrep1(i,n) for(int_fast32_t i = ((int_fast32_t)(n)); i >= 1; i--)   // 1-indexed降順
#define all(x) (x).begin(),(x).end()

using namespace std;

template<typename T>
using vec = vector<T>;
using i32 = int_fast32_t;
using i64 = int_fast64_t;
using u32 = uint_fast32_t;
using u64 = uint_fast64_t;
using ll = long long;
using ld = long double;
using vi = vec<int_fast32_t>;
using vl = vec<int_fast64_t>;
using vld = vec<ld>;
using vii = vec<vi>;  // 2次元配列
using PII = pair<int_fast32_t, int_fast32_t>;

template<class T>
using maxheap = std::priority_queue<T>;
template<class T>
using minheap = std::priority_queue<T, std::vector<T>, std::greater<T>>;

template<class T,class U> inline bool chmax(T &a, const U &b){if(a<b){a=b;return 1;}return 0;}
template<class T,class U> inline bool chmin(T &a, const U &b){if(a>b){a=b;return 1;}return 0;}

const ld Pi = std::acos(-1.0L);
constexpr ll infll = (1LL<<62)-1;
constexpr int inf = (1<<30)-1;
const int mod = 1000000007;
// const int mod = 998244353;


/*FUNCs=================================================*/


/*MAIN==================================================*/
signed main(){
    int START_TIME = clock();
    cin.tie(nullptr);
    ios::sync_with_stdio(false);   //cin cout 高速化
//    cout << fixed << setprecision(15);

    i32 n; cin >> n;
    vi a(n); rep(i,n) cin >> a[i];

    i32 ans = 0;
    for(i32 i = 0; i < n-1; ++i) {
        ans += min(a[i], a[i+1]);
        if(a[i] < a[i+1]) a[i+1] -= a[i], a[i] = 0;
        else a[i] -= a[i+1], a[i+1] = 0;

//        rep(i,n) cout << a[i] << (i == n-1 ? '\n' : ' ');
    }
    rep(i,n) ans += a[i]/2;

    cout << ans << '\n';

    // printf("ExecutionTime: %d /ms\n",1000*(int)((clock()-START_TIME)/CLOCKS_PER_SEC));
}