# 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():
	Px, Py = getlist()
	Qx, Qy = getlist()
	anspre = abs(Px - Qx) + abs(Py - Qy)
	print(anspre / 2)


if __name__ == '__main__':
	main()