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:
        i = peaks.pop()
        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) and j not in peaks:
                peaks.append(j)

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

print(result)