#include <iostream>
#include <algorithm>
#include <cstdio>
#include <string>

#define ll long long

using namespace std;

int main(){

   int px,py,qx,qy;
   cin>>px>>py>>qx>>qy;
   double m,n;
   printf("%.1lf\n",(double)(abs(px-qx)+abs(qy-py))/2);

   return 0;
}