using System;
using System.Linq;
using System.Collections.Generic;

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            AGC003.B();
        }

        class AGC003
        {
            public static void A()
            {
                var S = Read.Str();
                HashSet<char> hash = new HashSet<char>();
                foreach(var s in S)
                {
                    if (!hash.Contains(s)) { hash.Add(s); }
                }
                if (hash.Contains('N') && !hash.Contains('S')) { Console.WriteLine("No"); return; }
                if (hash.Contains('S') && !hash.Contains('N')) { Console.WriteLine("No"); return; }
                if (hash.Contains('E') && !hash.Contains('W')) { Console.WriteLine("No"); return; }
                if (hash.Contains('W') && !hash.Contains('E')) { Console.WriteLine("No"); return; }
                Console.WriteLine("Yes");
            }
            public static void B()
            {
                var N = Read.Int();
                int[] cards = Enumerable.Range(0, N + 1).Select(i => i < N ? Read.Int() : 0).ToArray();
                long count = 0;
                for (int i = 0; i < N; ++i)
                {
                    count += cards[i] / 2;
                    if (cards[i] % 2 == 1 && cards[i + 1] > 0)
                    {
                        count++;
                        cards[i + 1]--;
                    }
                }
                Console.WriteLine(count);
            }
            public static void C()
            {
                var N = Read.Int();
                var A = Enumerable.Range(0, N).Select(i => Read.Int()).ToArray();
                var ASort = A.OrderBy(x => x).ToArray();
                var index = new Dictionary<int, int>();
                for(int i = 0; i < N; ++i)
                {
                    index.Add(ASort[i], i);
                }

                int count = 0;
                for(int i = 0; i < N; i += 2)
                {
                    if (index[A[i]] % 2 == 1) { count++; }
                }
                Console.WriteLine(count);
            }
        }


        class Util
        {
        }

        static class Read
        {
            static Queue<string> queue = new Queue<string>();
            static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
            public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
            public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
            public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
            public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
            public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
            public static string[] StrN() { return Console.ReadLine().Split(' '); }
            public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
            public static long[,] LongMatrix(int r, int c)
            {
                var mat = new long[r, c];
                for (int i = 0; i < r; ++i)
                {
                    var x = Read.LongN();
                    for (int j = 0; j < c; ++j)
                    {
                        mat[i, j] = x[j];
                    }
                }
                return mat;
            }
            public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M)
            {
                Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new HashSet<int>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1]);
                    dict[ab[1]].Add(ab[0]);
                }
                return dict;
            }
            public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
            {
                Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new Dictionary<int, long>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1], ab[2]);
                    dict[ab[1]].Add(ab[0], ab[2]);
                }
                return dict;
            }
        }
    }
}


Measured memory_bytes: 16848