def check(nums):
    isDown=False
    once=False
    nums=[-math.inf]+nums+[math.inf]
    for i in range(1,len(nums)):
        if not isDown:
            if nums[i]<nums[i-1]:
                if once:
                    return False
                isDown=True
        elif isDown:
            if nums[i]>=nums[i-1] and nums[i-1]>=nums[i-3] :
                isDown=False
                once=True
            elif nums[i]>=nums[i-2] and nums[i-2]>=nums[i-3]:
                isDown=False
                once=True
            else:
                return False

                
    return True