#include <bits/stdc++.h>
using namespace std;

#define FOR(i,a,b) for(int i=a;i<b;i++)
#define REP(i,b) FOR(i,0,b)

using ll = long long;
const ll mod = LLONG_MAX;


int main(){
  double px,py,qx,qy;
  cin>>px>>py>>qx>>qy;
  double sa=abs(px-qx),sa2=abs(py-qy);
  cout<<fixed<<setprecision(1)<<(sa+sa2)/2<<endl;
  return 0;
}