from collections import deque

n = int(input())
a = list(map(int, input().split()))

queue = deque()

# Find initial peaks (0-based index)
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:
    i = queue.popleft()
    # Check if i is still a valid peak
    if i < 1 or i >= n-1:
        continue
    left = a[i-1]
    current = a[i]
    right = a[i+1]
    if left < current and current > right:
        # Perform the operation
        new_val = min(left, right) - 1
        a[i] = new_val
        count += 1
        # Check neighboring positions for new peaks
        for j in [i-1, i+1]:
            if 0 < j < n-1:
                # Check if j is a peak
                if a[j-1] < a[j] and a[j] > a[j+1]:
                    queue.append(j)

print(count)