from collections import deque

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

# Initialize the queue with all initial peaks
for i in range(1, n - 1):
    if A[i - 1] < A[i] and A[i] > A[i + 1]:
        queue.append(i)

count = 0

while queue:
    pos = queue.popleft()
    # Check if the current position is still a peak
    if pos < 1 or pos >= n - 1:
        continue
    if A[pos - 1] >= A[pos] or A[pos] <= A[pos + 1]:
        continue
    # Perform the operation
    m = min(A[pos - 1], A[pos + 1]) - 1
    A[pos] = m
    count += 1
    # Check neighboring positions for new peaks
    for delta in (-1, 1):
        new_pos = pos + delta
        if 1 <= new_pos < n - 1:
            if A[new_pos - 1] < A[new_pos] and A[new_pos] > A[new_pos + 1]:
                queue.append(new_pos)

print(count)