def max_operations():
    import sys
    input = sys.stdin.read().split()
    n = int(input[0])
    a = list(map(int, input[1:n+1]))
    
    count = 0
    i = 0
    while i < n-2:
        if a[i] < a[i+1] and a[i+1] > a[i+2]:
            # Perform the operation
            new_val = min(a[i], a[i+2]) - 1
            a[i+1] = new_val
            count += 1
            # After operation, check if i can still be a peak or if i-1 became a peak
            # We might need to check previous indices
            if i > 0 and a[i-1] < a[i] and a[i] > a[i+1]:
                i = max(i-1, 0)
                continue
            else:
                i = max(i-1, 0)
                continue
        i += 1
    
    print(count)

max_operations()