#define _USE_MATH_DEFINES	//M_PI(π),M_SQRT2(√2)
#include <iostream>			//std::cout, std::cin
#include <string>			//std::string,std::to_string(C++11)
#include <vector>			//std::vector
#include <valarray>			//std::valarray
#include <algorithm>		//std::sort
#include <time.h>			//localtime_s
#include <cstdlib>			//abs
#include <cmath>			//abs,std::pow,sqrt,sin,cos,round,floor,ceil
#include <fstream>			//std::ifstream,std::ofstream
#include <iomanip>			//std::setprecision,std::setw,std::setfill
#include <random>			//std::random(C++11)
#include <numeric>			//std::accumulate
#include <functional>		//std::greater

int main(void) {

	//test用
	//std::ifstream in("123.txt");
	//std::cin.rdbuf(in.rdbuf());

	int Px = 0, Py = 0, Qx = 0, Qy = 0;
	std::cin >> Px >> Py >> Qx >> Qy;

	std::cout << std::fixed << std::setprecision(2) << (abs(Px - Qx) + abs(Py - Qy)) / 2.0 << std::endl;


}