# -*- coding: utf-8 -*-



import sys



def input(): return sys.stdin.readline().strip()

def list2d(a, b, c): return [[c] * b for i in range(a)]

def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]

def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]

def ceil(x, y=1): return int(-(-x // y))

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]

def Yes(): print('Yes')

def No(): print('No')

def YES(): print('YES')

def NO(): print('NO')

sys.setrecursionlimit(10 ** 9)

INF = float('inf')

MOD = 10 ** 9 + 7



N = INT()

A = LIST(N)



ans = A[0] // 2

mod = A[0] % 2

for i in range(1, N):

    ans += A[i] // 2

    tmpmod = A[i] % 2

    if mod == tmpmod == 1:

        ans += 1

        mod = 0

    elif tmpmod == 1:

        mod = 1

    elif A[i] == 0:

        mod = 0

print(ans)

# -*- coding: utf-8 -*-



import sys



def input(): return sys.stdin.readline().strip()

def list2d(a, b, c): return [[c] * b for i in range(a)]

def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]

def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]

def ceil(x, y=1): return int(-(-x // y))

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]

def Yes(): print('Yes')

def No(): print('No')

def YES(): print('YES')

def NO(): print('NO')

sys.setrecursionlimit(10 ** 9)

INF = float('inf')

MOD = 10 ** 9 + 7



N = INT()

A = LIST(N)



ans = cnt = 0

for i in range(N):

    if A[i] == 0:

        ans += cnt // 2

        cnt = 0

    else:

        cnt += A[i]

ans += cnt // 2



print(ans)

# -*- coding: utf-8 -*-
import sys


def input():
    return sys.stdin.readline().strip()


def list2d(a, b, c):
    return [[c] * b for i in range(a)]


def list3d(a, b, c, d):
    return [[[d] * c for j in range(b)] for i in range(a)]


def list4d(a, b, c, d, e):
    return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]


def ceil(x, y=1):
    return int(-(-x // y))


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST(N=None):
    return list(MAP()) if N is None else [INT() for i in range(N)]


def Yes():
    print("Yes")


def No():
    print("No")


def YES():
    print("YES")


def NO():
    print("NO")


sys.setrecursionlimit(10**9)
INF = float("inf")
MOD = 10**9 + 7
N = INT()
A = LIST(N)
ans = A[0] // 2
mod = A[0] % 2
for i in range(1, N):
    ans += A[i] // 2
    tmpmod = A[i] % 2
    if mod == tmpmod == 1:
        ans += 1
        mod = 0
    elif tmpmod == 1:
        mod = 1
    elif A[i] == 0:
        mod = 0
print(ans)

# -*- coding: utf-8 -*-
import sys


def input():
    return sys.stdin.readline().strip()


def list2d(a, b, c):
    return [[c] * b for i in range(a)]


def list3d(a, b, c, d):
    return [[[d] * c for j in range(b)] for i in range(a)]


def list4d(a, b, c, d, e):
    return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]


def ceil(x, y=1):
    return int(-(-x // y))


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST(N=None):
    return list(MAP()) if N is None else [INT() for i in range(N)]


def Yes():
    print("Yes")


def No():
    print("No")


def YES():
    print("YES")


def NO():
    print("NO")


sys.setrecursionlimit(10**9)
INF = float("inf")
MOD = 10**9 + 7
N = INT()
A = LIST(N)
ans = cnt = 0
for i in range(N):
    if A[i] == 0:
        ans += cnt // 2
        cnt = 0
    else:
        cnt += A[i]
ans += cnt // 2
print(ans)

-ans = A[0] // 2 -mod = A[0] % 2 -for i in range(1, N): -    ans += A[i] // 2 -    tmpmod = A[i] % 2 -    if mod == tmpmod == 1: -        ans += 1 -        mod = 0 -    elif tmpmod == 1: -        mod = 1 -    elif A[i] == 0: -        mod = 0 +ans = cnt = 0 +for i in range(N): +    if A[i] == 0: +        ans += cnt // 2 +        cnt = 0 +    else: +        cnt += A[i] +ans += cnt // 2