#include <iostream>
#include <algorithm>

using namespace std;

int main() {
  int px, py, qx, qy;
  cin >> px >> py >> qx >> qy;

  double ans=double(max(px, qx)-min(px, qx)+max(py, qy)-min(py, qy))/2;

  printf("%lf\n", ans);
  return 0;
}