def max_operations():
    import sys
    input = sys.stdin.read().split()
    n = int(input[0])
    a = list(map(int, input[1:n+1]))
    
    ops = 0
    i = 0
    while i <= n-3:
        if a[i] < a[i+1] and a[i+1] > a[i+2]:
            min_val = min(a[i], a[i+2])
            a[i+1] = min_val - 1
            ops += 1
            i = max(i-1, 0)  # 检查前面的位置是否满足条件
        else:
            i += 1
    print(ops)

max_operations()