#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
using namespace std;
typedef long long ll;
#define int ll

signed main(void) {
  double px, py, qx, qy;

  cin >> px >> py;
  cin >> qx >> qy;

  double ans, tmp1, tmp2;
  tmp1 = (px > qx) ? (px - qx) : (qx - px);
  tmp2 = (py > qy) ? (py - qy) : (qy - py);
  //cout << tmp1 << " " << tmp2 << endl;
  ans = (tmp1 + tmp2) / 2.0;

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