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

int main() {
	float Px, Py, Qx, Qy, l, t;

	scanf("%f %f", &Px, &Py);
	scanf("%f %f", &Qx, &Qy);

	l = fabsf(Px - Qx) + fabsf(Py - Qy);
	
	printf("%f\n", l / 2);

	return 0;
}