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
    
    q = deque()
    for i in range(N-2):
        if A[i] < A[i+1] and A[i+1] > A[i+2]:
            q.append(i)
    
    operations = 0
    
    while q:
        i = q.popleft()
        left = A[i]
        right = A[i+2]
        min_val = min(left, right)
        A[i+1] = min_val - 1
        operations += 1
        
        # Check i-1
        if i - 1 >= 0:
            j = i - 1
            if j <= N - 3:
                if A[j] < A[j+1] and A[j+1] > A[j+2]:
                    q.append(j)
        
        # Check i+1
        j = i + 1
        if j <= N - 3:
            if A[j] < A[j+1] and A[j+1] > A[j+2]:
                q.append(j)
    
    print(operations)

if __name__ == "__main__":
    main()