#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <string>
#include <cstdio>
#include <cmath>

using namespace std;

int main() {

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

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

	printf("%.15f\n",ans);

	return 0;
}