#include <bits/stdc++.h>
using namespace std;
 
int main(){
  int x1,x2,y1,y2;
  cin>>x1>>y1>>x2>>y2;
  int X=abs(x2-x1)+abs(y2-y1);
  if(X%2==0){
      cout<<X/2<<endl;
  }
  else{
      cout<<(double)X/2<<endl;
      }
}