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

int main(void){
	double px, py, qx, qy; cin >> px >> py >> qx >> qy;
	double sum = abs(px - qx) + abs(py - qy);
	printf("%f\n", sum / 2.0);
	return 0;
}