#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=q_x-p_x, y=q_y-p_y;
    if(x<0) { x*=-1; }
    if(y<0) { y*=-1; }
    printf("%f\n",(x+y)/2);
}