#include <iostream>
#include <algorithm>
#include <vector>
#include <cmath>
#include <set>
#include <map>
#include <iomanip>
#include <cassert>
#include <stack>
#include <queue>
#include <deque>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
//using namespace __gnu_pbds;

typedef long long ll;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;

// template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
//  order_of_key (k) : Number of items strictly smaller than k .
//  find_by_order(k) : K-th element in a set (counting from zero).
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define pb push_back
#define ppb pop_back
#define mkp make_pair
#define F first
#define S second
#define show(a) cerr << #a <<" -> "<< a <<"\n"
#define fo(a, b, c, d) for(int (a) = (b); (a) <= (c); (a) += (d))
#define foo(a, b, c ,d) for(int (a) = (b); (a) >= (c); (a) -= (d))
//#define int ll

const int N = 2e5 + 5;
const int oo = 1e9 + 5;

int n, a[N];
ll ans;

void solve() {
	cin >> n;
	for (int i = 1; i <= n; ++i) {
		cin >> a[i];
	}	
	for (int i = 1; i <= n; ++i) {
		if (i < n && (a[i] & 1) && (a[i+1] > 0)) {
			/**int mx = min(a[i], a[i + 1]);
			ans += mx;
			a[i] -= mx;
			a[i+1] -= mx;
			ans += a[i] / 2;**/
			++ans;
			--a[i];
			--a[i+1]; 
		}
		ans += a[i] / 2;
	}
	cout << ans;
}

 main () {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	int tt = 1;
	while (tt --) {
		solve();
	}
	return 0;
}
/*
	“I’m not gonna run away, I never go back on my word! That’s my nindo: my ninja way.” Naruto

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*/

Measured memory_bytes: 640