INF = 1000000000000000000

def calc(mode_num, mode_sum, mode_freq, lower, upper, all_num) :
	other = all_num - mode_num * mode_freq
	if not mode_num :
		if not mode_sum :
			other_block = other // (mode_freq - 1)
			other_leftover = other % (mode_freq - 1)
			if other_block + (1 if other_leftover else 0) > upper - lower + 1 : return -INF
			return other_block * (upper + upper - other_block + 1) // 2 * (mode_freq - 1) + \
				other_leftover * (upper - other_block)
		else : return -INF
	adding = mode_sum - mode_num * (lower + lower + mode_num - 1) // 2
	other_block = other // (mode_freq - 1)
	other_leftover = other % (mode_freq - 1)
	if other_block and not other_leftover :
		other_block -= 1
		other_leftover = mode_freq - 1
	if other_block + (1 if other_leftover else 0) > upper - lower + 1 : return -INF
	default_clearance = upper - lower + 1 - (other_block + (1 if other_leftover else 0) + mode_num)
	slide = adding // mode_num - default_clearance
	sum = mode_sum * mode_freq + other_block * (upper + upper - other_block + 1) // 2 * (mode_freq - 1) + \
		other_leftover * (upper - other_block)
	if slide > 0 :
		sum -= (other_leftover + (mode_freq - 1) * (slide - 1)) * mode_num
		sum -= adding % mode_num * (mode_freq - 1)
	elif not slide : sum -= adding % mode_num * other_leftover
	return sum

def get_candidate(mode_num, mode_freq, upper, all_num) :
	other = all_num - mode_num * mode_freq
	if not mode_num : return 0
	other_used = other // (mode_freq - 1)
	if other % (mode_freq - 1) : other_used += 1
	return (upper - other_used + upper - other_used - mode_num + 1) * mode_num // 2

def solve_sub(median, median_l, median_r, freq, mode_all_sum, left, right) :
	lower = 0
	upper = mode_all_sum
	lower = max(lower, (left - 1) * left // 2)
	upper = min(upper, (median - 1 + median - left) * left // 2)
	lower = max(lower, mode_all_sum - (p + p - right + 1) * right // 2)
	upper = min(upper, mode_all_sum - (median + 1 + median + right) * right // 2)
	candidates = [lower, upper]
	candidates.append(get_candidate(left, freq, median - 1, median_l))
	candidates.append(candidates[-1] + left)
	candidates.append(mode_all_sum - get_candidate(right, freq, p, n - median_r))
	candidates.append(candidates[-1] - right)
	res = -1
	for left_sum in candidates :
		if left_sum >= lower and left_sum <= upper :
			cur = (median_r - median_l) * median
			cur += calc(left, left_sum, freq, 0, median - 1, median_l)
			cur += calc(right, mode_all_sum - left_sum, freq, median + 1, p, n - median_r)
			res = max(res, cur)
	return res


for testcnt in range(10) :
	n, p, median, mode = map(int, input().split())
	half = n // 2
	upper = p - median
	res = -1
	if median >= half and median + half <= p :
		min_sum = median + (half - 1) * half // 2 + (median + 1 + median + half) * half // 2
		max_sum = median + (median - 1 + median - half) * half // 2 + (p + p - half + 1) * half // 2;
		if mode * n >= min_sum and mode * n <= max_sum : res = mode * n

	for freq in range(2, n + 1) :
		for left in range(0, min(median, half // freq) + 1) :
			for right in range(0, min(upper, half // freq) + 1) :
				median_l_min = left * freq
				median_l_max = min(half, left * freq + (median - left) * (freq - 1))
				median_r_max = n - right * freq
				median_r_min = max(half + 1, n - right * freq - (upper - right) * (freq - 1))
				if left or right :
					mode_all_sum = mode * (left + right)
					median_r = median_r_min
					median_l = max(median_l_min, median_r - (freq - 1))
					if median_l <= half :
						res = max(res, solve_sub(median, median_l, median_r, freq, mode_all_sum, left, right))
				median_l_min = max(median_l_min, median_r_min - freq)
				median_l_max = min(median_l_max, median_r_max - freq)
				if median_l_max >= median_l_min :
					median_l = median_l_min
					mode_all_sum = mode * (left + right + 1) - median
					res = max(res, solve_sub(median, median_l, median_l + freq, freq, mode_all_sum, left, right))

	print(res)