import sys
from collections import deque

def main():
    input = sys.stdin.read().split()
    N = int(input[0])
    A = list(map(int, input[1:N+1]))
    
    if N < 3:
        print(0)
        return
    
    # Convert to 0-based index
    queue = deque()
    for i in range(1, N-1):
        if A[i-1] < A[i] and A[i] > A[i+1]:
            queue.append(i)
    
    operation_count = 0
    
    while queue:
        i = queue.popleft()
        
        if not (A[i-1] < A[i] and A[i] > A[i+1]):
            continue
        
        new_val = min(A[i-1], A[i+1]) - 1
        A[i] = new_val
        operation_count += 1
        
        # Check left triplet (i-2, i-1, i)
        if i - 1 >= 1:
            if A[i-2] < A[i-1] and A[i-1] > A[i]:
                queue.append(i-1)
        
        # Check right triplet (i, i+1, i+2)
        if i + 1 <= N-2:
            if A[i] < A[i+1] and A[i+1] > A[i+2]:
                queue.append(i+1)
    
    print(operation_count)

if __name__ == '__main__':
    main()