import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
# md = 10**9+7
md = 998244353

from random import randrange
from heapq import *

def left(aa):
    while 1:
        for i in range(n-2):
            if aa[i] < aa[i+1] and aa[i+1] > aa[i+2]:
                aa[i+1] = min(aa[i], aa[i+2])-1
                print(i, aa)
                break
        else:
            break

def mx(aa):
    hp = []
    ans = 0
    for i, a in enumerate(aa):
        if i == 0 or i == n-1: continue
        if aa[i-1] < a and a > aa[i+1]:
            heappush(hp, (-a, i))
    while hp:
        a, i = heappop(hp)
        if aa[i] != -a: continue
        ans += 1
        aa[i] = min(aa[i-1], aa[i+1])-1
        if i-2 >= 0 and aa[i-2] < aa[i-1]: heappush(hp, (-aa[i-1], i-1))
        if i+2 < n and aa[i+1] > aa[i+2]: heappush(hp, (-aa[i+1], i+1))
        # print(i,aa)
    return ans

n = II()
aa = LI()

# aa=[randrange(1,50) for _ in range(n)]
# for i in range(n-1):
#     if aa[i]==aa[i+1]:
#         aa[i+1]+=1

# print(aa)

# left(aa[:])
# print()
ans = mx(aa[:])
print(ans)