import sys
from collections import deque

def main():
    n = int(sys.stdin.readline())
    a = list(map(int, sys.stdin.readline().split()))
    if n < 3:
        print(0)
        return
    q = deque()
    for i in range(n-2):
        if a[i] < a[i+1] and a[i+1] > a[i+2]:
            q.append(i)
    count = 0
    while q:
        i = q.popleft()
        if i < 0 or i >= n-2:
            continue
        if a[i] < a[i+1] and a[i+1] > a[i+2]:
            count += 1
            new_val = min(a[i], a[i+2]) - 1
            a[i+1] = new_val
            for next_i in [i-1, i+1]:
                if 0 <= next_i <= n-3:
                    q.append(next_i)
    print(count)

if __name__ == "__main__":
    main()