#include <iostream>
#include <cstdio>
using namespace std;

int abs(int x) {
	if (x < 0) return -x;
	return x;
}

int main() {
	int px, py;
	int qx, qy;
	
	cin >> px >> py >> qx >> qy;
	double t = (abs(px - qx) + abs(py - qy)) * 0.5;
	printf("%.14f\n", t);
	return 0;
}