import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	A, B, C, D = getlist()
	if B >= C:
		ans = (D - C + 1) * (B - A + 1) - (B - C + 1)
	else:
		ans = (D - C + 1) * (B - A + 1)

	print(ans)
	
if __name__ == '__main__':
	main()