from collections import deque

def main():
    import sys
    input = sys.stdin.read().split()
    N = int(input[0])
    A = list(map(int, input[1:N+1]))
    
    if N < 3:
        print(0)
        return
    
    dq = deque()
    
    for i in range(N-2):
        if A[i] < A[i+1] and A[i+1] > A[i+2]:
            dq.append(i)
    
    count = 0
    
    while dq:
        i = dq.popleft()
        
        if i < 0 or i >= N - 2:
            continue
        
        if not (A[i] < A[i+1] and A[i+1] > A[i+2]):
            continue
        
        new_val = min(A[i], A[i+2]) - 1
        A[i+1] = new_val
        count += 1
        
        if i - 1 >= 0:
            if (i - 1) <= N - 3:
                if A[i-1] < A[i] and A[i] > A[i+1]:
                    dq.append(i-1)
        
        if i + 1 <= N - 3:
            if A[i+1] < A[i+2] and A[i+2] > A[i+3]:
                dq.append(i+1)
    
    print(count)

if __name__ == "__main__":
    main()