#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
typedef long long lint;

#define MIN(a,b) ((a)<(b)?(a):(b))
#define MAX(a,b) ((a)>(b)?(a):(b))


int main()
{
	int px, py, qx, qy;
	cin >> px >> py >> qx >> qy;
	printf("%.3f\n", (double)(abs(px - qx) + abs(py - qy)) / 2);
	
	return 0;
}