#define _USE_MATH_DEFINES

#include <iostream>	//cin, cout
#include <vector>	//vector
#include <algorithm> //sort,min,max,count
#include <string>	//string,getline
#include <ios>		//fixed
#include <iomanip>	//setprecision
#include <utility> //swap
#include <cstdlib>	//abs(int)
#include <cmath>	//sqrt
#include <sstream>	//stringstream,getline
#include <cmath>	//ceil,M_PI

using namespace std;

int main() {

	int px, py, qx, qy;
	cin >> px >> py >> qx >> qy;

	double ans = abs((px + qx) / 2.0 - px) + abs((py + qy) / 2.0 - py);

	cout << ans << endl;

	return 0;

}