#include <iostream>
#include <iomanip>
int main(void){
  int px,py,qx,qy;
  float d;
  std::cin >> px >>py;
  std::cin >> qx >>qy;

  //std::cout << abs(px-qx) <<std::endl;
  //std::cout << abs(py-qy) <<std::endl;
  //std::cout << abs(px-qx)+abs(py-qy) <<std::endl;
  d=abs(px-qx)+abs(py-qy);
  //std::cout << d <<std::endl;
  std::cout << std::setprecision(20) << d/2.0 <<std::endl;
  
  
  //d=(abs(px-qx)+abs(py-qy))/2.0;
 //std::cout << d << std::endl;
}