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

int main() {
	double a,b,c,d;
	cin>>a>>b>>c>>d;
	a-=c;b-=d;
	if(a<0)a*=-1.0;
	if(b<0)b*=-1.0;
	cout<<fixed<<setprecision(2)<<a/2+b/2<<endl;
	return 0;
}