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// C++ Conpetitive Programming Template 
// Ver 1.1 (codename: Hinata)
// Last Modified : 2019-05-28
// Created       : 2019-05-25
// Auther        : shirataki3
#include <bits/stdc++.h>
using namespace std;
// typename alias
template <class T> using vec = vector<T>;
template <class T, class U> using vep = vec<pair<T,U> >;
template <class T> using PQ = priority_queue<T>;
using ll = long long;
using vll = vec<ll>;
using pll = pair<ll, ll>;
using vpll = vec<pll>;
using str = string;
// あくまのちから
//#define int ll

// const
static const long long INF = 1LL<<59;
static const long long MOD = (ll)1e9+7;
static const double PI = 3.14159265358979323846;

// accelerate stdin, stdout
#define endl "\n"
#define fast_io ios_base::sync_with_stdio(false);cin.tie(NULL)
// repeat macro
#define FOR(i, a, b) for(ll (i)=(a);(i)<=(b);i++)
#define rFOR(i, a, b) for(ll (i)=(b);(i)>=(a);i--)
#define REP(i, n) for(ll i=0;i<n;i++)
// vector macro
#define WHOLE(v) (v).begin(), (v).end()
#define ASC(v) sort(WHOLE(v))
#define DESC(v) sort((v).rbegin(), (v).rend())
#define INV(v) reverse(WHOLE(v))
#define FOLDL(src, dst, lambda) partial_sum(WHOLE(src), (dst).begin(), lambda)
#define FOLDR(src, dst, lambda) INV(src);partial_sum(WHOLE(src), (dst).begin(), lambda);INV(dst)
#define CUMSUML(src, dst) FOLDL(src, dst, [](auto a, auto b) {return a+b;})
#define CUMSUMR(src, dst) FOLDR(src, dst, [](auto a, auto b) {return a+b;})
#define ACCUM(v, d0, lambda) accumulate(WHOLE(v), d0, lambda)
#define SUM(v, d0) ACCUM(v, d0, [](auto a, auto b) {return a+b;})
#define ERASE(v, i) v.erase(v.begin() + i)
#define UNIQ(v) ASC(v);v.erase(unique(v.begin(), v.end()), v.end())

// io helper
template <typename T>void debug(T t) {cerr << t << endl;}
template <typename T, typename ...Args>void debug(T t, Args ...args) {cerr << t << " ";debug(args...);}
template <typename T>void out(T t) {cout << t << endl;}
template <typename T, typename ...Args>void out(T t, Args ...args) {cout << t << " ";out(args...);}
template <typename T>istream& operator>>(istream &is, vec<T> &v) {for(auto &a:v) is >> a; return is;}
template <typename T>ostream& operator<<(ostream &os, vec<T> &v) {os << "[";REP(i, v.size() - 1) os << v[i] <<", "; os << v[v.size()-1]<<"]";return os;}
template <typename T, typename U>ostream& operator<<(ostream &os, pair<T, U> &p) {os << "(" << p.first << ", " << p.second << ")";return os;}
template <typename T, typename U>ostream& operator<<(ostream &os, map<T, U> &mp) {os << "{ ";for(auto &p:mp) os << p.first << ": " << p.second << ", "; os << "}";return os;}

void solve() {
    ll N;
    cin >> N;
    vll A(N);
    cin >> A;
    A.push_back(0);
    ll ans = 0;
    REP(i, N) {
        ans += (A[i] / 2);
        A[i] %= 2;
        if(A[i] == 1 && A[i + 1] >= 1) {
            ans++;
            A[i]--;A[i + 1]--;
        }
    }
    out(ans);
}

signed main() {
    fast_io;
    cout << setprecision(20) << fixed;
    solve();
    return 0;
}

Measured memory_bytes: 1792