#include<bits/stdc++.h> using namespace std; using Int = long long; struct Precision{ Precision(){ cout<<fixed<<setprecision(12); } }precision_beet; //INSERT ABOVE HERE signed main(){ Int px,py,qx,qy; cin>>px>>py>>qx>>qy; double ans=0; ans+=abs(px-qx)/2.; ans+=abs(py-qy)/2.; //if((abs(px-qx)&1)&&(abs(py-qy)&1)) ans+=0.5; cout<<ans<<endl; return 0; }