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

int main(void)
{
  int px, py, qx, qy;
  cin >> px >> py >> qx >> qy;
  
  int x = px - qx, y= py - qy;
  if(x<0) x *= -1;
  if(y<0) y *= -1;
  double ans = (x+y)/2.0;

  printf("%.1f\n", ans);
  return 0;
}