#include <bits/stdc++.h>

int main() {
  int px, py, qx, qy;
  scanf("%d", &px);
  scanf("%d", &py);
  scanf("%d", &qx);
  scanf("%d", &qy);

  printf("%lf\n", 0.5 * (std::abs(px - qx) + std::abs(py - qy))); 
  
  return 0;
}