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

double abs(double x){return (x >= 0 ? x : -x);}

int main(){
	int Px,Py,Qx,Qy;
	cin >> Px >> Py >> Qx >> Qy;
	cout << fixed << setprecision(10) << (abs(Px - Qx) + abs(Py - Qy)) / 2. << endl;
}