#include<iostream>

using namespace std;

int main()
{
    double p_x,p_y,q_x,q_y;
    cin>>p_x>>p_y>>q_x>>q_y;
    
    double x = p_x-q_x, y=p_y-q_y;
    if(x<0) { x*=-1; }
    if(y<0) { y*=-1; }
    
    cout << x/2+y/2 << endl;

}