#include <bits/stdc++.h>
#define rep(i,n) for(ll (i)=0;(i)<(n);(i)++)
#define repi(i,a,b) for(ll (i)=(a);(i)<(b);(i)++)
#define SIZE_OF_ARRAY(array)    (sizeof(array)/sizeof(array[0]))
typedef long long ll;
using namespace std;
int main(){
  //マンハッタン距離
  double px,py,qx,qy;
  double ans;
  cin >> px >> py >> qx >> qy;
  ans = (abs(px-qx)+abs(py-qy))*0.5;
  cout << ans << endl;
  return 0;
  
}