from collections import deque

n = int(input())
A = list(map(int, input().split()))
queue = deque()

# 初始化队列，找到所有初始的峰的位置
for j in range(1, n - 1):
    if A[j-1] < A[j] and A[j] > A[j+1]:
        queue.append(j)

count = 0

while queue:
    j = queue.popleft()
    # 检查当前j是否有效并满足峰的条件
    if j < 1 or j >= len(A) - 1:
        continue
    if not (A[j-1] < A[j] and A[j] > A[j+1]):
        continue
    # 执行操作
    min_val = min(A[j-1], A[j+1])
    A[j] = min_val - 1
    count += 1
    # 检查相邻的位置是否形成新的峰
    for neighbor in [j-1, j+1]:
        if 1 <= neighbor <= len(A) - 2:
            if A[neighbor-1] < A[neighbor] and A[neighbor] > A[neighbor+1]:
                queue.append(neighbor)

print(count)