def count_operations(N, A):
    def is_peak(i):
        return 0 < i < N - 1 and A[i - 1] < A[i] > A[i + 1]

    peaks = {i for i in range(1, N - 1) if is_peak(i)}
    count = 0

    while peaks:
        to_check = list(peaks)
        for i in to_check:
            if i in peaks:
                peaks.remove(i)
                count += 1
                A[i] = min(A[i - 1], A[i + 1]) - 1
                for j in (i - 1, i + 1):
                    if 0 < j < N - 1 and is_peak(j):
                        peaks.add(j)

    return count

N = int(input())
A = list(map(int, input().split()))
result = count_operations(N, A)

print(result)