from collections import deque

def main():
    import sys
    input = sys.stdin.read
    data = input().split()
    N = int(data[0])
    A = list(map(int, data[1:N+1]))
    
    dq = deque()
    for i in range(1, N-1):
        if A[i-1] < A[i] and A[i] > A[i+1]:
            dq.append(i)
    
    count = 0
    while dq:
        i = dq.popleft()
        if i < 1 or i >= N-1:
            continue
        if A[i-1] < A[i] and A[i] > A[i+1]:
            new_val = min(A[i-1], A[i+1]) - 1
            A[i] = new_val
            count += 1
            dq.append(i-1)
            dq.append(i+1)
    
    print(count)

if __name__ == "__main__":
    main()