#!/usr/bin/env python
#coding:utf8

def read():
    x1, y1 = map(int, raw_input().split())
    x2, y2 = map(int, raw_input().split())
    return x1, y1, x2, y2


def work((x1, y1, x2, y2)):
    ans = 0
    ans += abs(x2 - x1) / 2.0
    ans += abs(y2 - y1) / 2.0
    print ans


if __name__ == "__main__":
    work(read())