#include <bits/stdc++.h>
#include <random>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;
//#define int ll
//#define int __int128
//#define ll __int128
typedef pair<int, int   > pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<pii> vpi;
typedef vector< vi > vvi;
typedef vector< vvi > vvvi;
typedef vector<short> vs;
typedef vector<vs> vvs;
typedef vector<vvs> vvvs;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
typedef vector<ld> vld;
typedef vector<vld> vvld;
typedef vector<vvld> vvvld;
typedef vector<string> vst;
typedef vector<vst> vvst;
typedef pair<ld, ld> pld;
//typedef complex<double> base;

#define inmin(a, b) a = min(a, (b))
#define inmax(a, b) a = max(a, (b))
#define ALL(a) a.begin(),a.end()
#define RALL(a) a.rbegin(),a.rend()
#define sqr(x) ((x) * (x))
#define fori(i, n) for(int i = 0; i < (int)(n); ++i)
#define SZ(a) ((int)((a).size()))
#define triple(T) tuple<T, T, T>
#define quad(T) tuple<T, T, T, T>
#define watch(x) cerr << (#x) << " = " << (x) << endl;
#define mp(a, b) make_pair(a, b)

#ifdef MAX_HOME
#define cerr cout
#else
#define cerr if (false) cerr
#endif

const double PI = 2 * acos(0.0);
#define rand shittttty_shit
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng_64(chrono::steady_clock::now().time_since_epoch().count());

const string DIGITS = "0123456789";
const string ALPH = "abcdefghijklmnopqrstuvwxyz";

template <class T0, class T1>
inline ostream & operator << (ostream &out, pair<T0, T1> &a) {
    return out << "{" << a.first << ", " << a.second << "}";
}

template <class T0, class T1, class T2>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << "}";
}

template <class T0, class T1, class T2, class T3>
inline ostream & operator << (ostream &out, tuple<T0, T1, T2, T3> &a) {
    return out << "{" << get<0>(a) << ", " << get<1>(a) << ", " << get<2>(a) << ", " <<  get<3>(a) << "}";
}

template<class T>
inline ostream & operator << (ostream &out, vector<T> &a) {
    out << "[";
    fori (i, a.size())
        out << a[i] << vector<string>{", ", "]  "}[i + 1 == a.size()];
    return out;
}

ostream & operator << (ostream &out, pii a) {
    cout << a.first << " " << a.second ;
    return out;
}

void smain();

signed main() {

#ifdef MAX_HOME
    freopen("input.txt", "r", stdin);
    clock_t start = clock();
#endif
    ios::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    cout << setprecision(12) << fixed;
    smain();
#ifdef MAX_HOME
    cout << "\n\n\n\nTOTAL EXECUTION TIME: " << float( clock () - start ) /  CLOCKS_PER_SEC << endl;
#endif
    return 0;
}

const int N = 1e5 + 100;

ll a[N];
ll dp[N][2];

void smain() {

    int n;
    cin >> n;
    fori (i, n) {
        cin >> a[i];
    }
    dp[0][0] = a[0] / 2;
    dp[0][1] = (a[0] - 1) / 2;
    for (int i = 0; i < n; ++i) {
        /* even */;
        if (i) {
            ll cur = a[i] / 2;
            inmax(dp[i][0], cur + dp[i - 1][0]);
            if (a[i] & 1) {
                if (a[i - 1] > 0)
                    inmax(dp[i][0], cur + dp[i - 1][1] + 1);
                inmax(dp[i][1], cur + dp[i - 1][0]);
            } else {
                if (a[i] > 0) {
                    inmax(dp[i][1], dp[i - 1][1] + cur);
                    inmax(dp[i][1], dp[i - 1][0] + cur - 1);
                }
            }
        }
//        fori (z, 2) {
//            cerr << "dp[" << i << "][" << z << "] = " << dp[i][z] << endl;
//        }
    }
    cout << max(dp[n - 1][0], dp[n - 1][1]);
}

Measured memory_bytes: 2560